Method of measuring a target, and metrology apparatus

ABSTRACT

Disclosed is a method of measuring a target, and a metrology apparatus. In one arrangement the target comprises a layered structure. The layered structure has a first target structure in a first layer and a second target structure in a second layer. The method comprises illuminating the target with measurement radiation using an illumination profile in the illumination pupil (u) that is offset from an imaginary line (IL) in the illumination pupil passing through the optical axis, to allow propagation to a detection region of the detection pupil of an allowed order (v2, v4) of a predetermined diffraction order while limiting propagation to the detection region of an equal and opposite order (v1′, v3′) of that predetermined diffraction order. Scattered radiation of plural double-diffracted allowed diffraction orders (w2, w4) is detected. A characteristic of the lithographic process is calculated using the detected scattered radiation of the predetermined diffraction orders.

BACKGROUND Field of the Invention

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. In lithographic processes, itis desirable frequently to make measurements of the structures created,e.g., for process control and verification. Various tools for makingsuch measurements are known, including scanning electron microscopes,which are often used to measure critical dimension (CD), and specializedtools to measure overlay, a measure of the accuracy of alignment of twolayers in a device. Overlay may be described in terms of the degree ofmisalignment between the two layers, for example reference to a measuredoverlay of 1 nm may describe a situation where two layers are misalignedby 1 nm.

Recently, various forms of scatterometers have been developed for use inthe lithographic field. These devices direct a beam of radiation onto atarget and measure one or more properties of the scatteredradiation—e.g., intensity at a single angle of reflection as a functionof wavelength; intensity at one or more wavelengths as a function ofreflected angle; or polarization as a function of reflected angle—toobtain a “spectrum” from which a property of interest of the target canbe determined. Determination of the property of interest may beperformed by various techniques: e.g., reconstruction of the target byiterative approaches such as rigorous coupled wave analysis or finiteelement methods; library searches; and principal component analysis.

The targets used by conventional scatterometers are relatively large,e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spotthat is smaller than the grating (i.e., the grating is underfilled).This simplifies mathematical reconstruction of the target as it can beregarded as infinite. However, in order to reduce the size of thetargets, e.g., to 10 μm by 10 μm or less, e.g., so they can bepositioned in amongst product features, rather than in the scribe lane,metrology has been proposed in which the grating is made smaller thanthe measurement spot (i.e., the grating is overfilled). Typically, suchtargets are measured using dark field scatterometry in which the zerothorder of diffraction (corresponding to a specular reflection) isblocked, and only higher orders processed. Examples of dark fieldmetrology can be found in international patent applications WO2009/078708 and WO 2009/106279 which documents are hereby incorporatedby reference in their entirety. Further developments of the techniquehave been described in patent publications US20110027704A,US20110043791A and US20120242970A. The contents of all theseapplications are also incorporated herein by reference.Diffraction-based overlay using dark-field detection of the diffractionorders enables overlay measurements on smaller targets. These targetscan be smaller than the illumination spot and may be surrounded byproduct structures on a wafer. Targets can comprise multiple gratingswhich can be measured in one image.

In the known metrology technique, overlay measurement results areobtained by measuring an overlay target twice under certain conditions,while either rotating the overlay target or changing the illuminationmode or imaging mode to obtain separately the −1st and the +1stdiffraction order intensities. The intensity asymmetry, a comparison ofthese diffraction order intensities, for a given overlay target providesa measurement of target asymmetry, that is asymmetry in the target. Thisasymmetry in the overlay target can be used as an indicator of overlayerror (undesired misalignment of two layers).

It has been found that changes in the manufacturing process of thesemiconductor device can reduce the robustness or reliability of overlayerror measurements.

It is an object of the invention to improve the robustness orreliability of measurements of a lithographic characteristic such asoverlay error.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of measuring a target formed by a lithographic process, thetarget comprising a layered structure having a first target structure ina first layer and a second target structure in a second layer, themethod comprising:

positioning the target in an optical axis of an optical system having anillumination pupil and a detection pupil in corresponding pupil planesof the optical system,

illuminating the target with measurement radiation using an illuminationprofile in the illumination pupil that is offset from an imaginary linein the illumination pupil passing through the optical axis, theimaginary line corresponding to a direction of periodicity of a targetstructure, wherein the illumination profile is configured to allowpropagation to a detection region of the detection pupil of an allowedorder of a predetermined diffraction order while limiting propagation tothe detection region of an equal and opposite order of thatpredetermined diffraction order;

detecting scattered radiation of plural allowed diffraction orders,wherein the allowed diffraction orders are generated by diffraction ofthe measurement radiation from the first target structure and aresubsequently diffracted from the second target structure; and

calculating a characteristic of the lithographic process using thedetected scattered radiation of the allowed diffraction orders.

According to a second aspect of the present invention, there is provideda metrology apparatus comprising:

an illumination system configured to illuminate with measurementradiation a target produced using a lithographic process on a substrate;and

a detection system configured to detect scattered radiation arising fromillumination of the target, wherein:

the metrology apparatus is operable to perform the method of the firstaspect.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the invention;

FIGS. 3(a)-3(d) comprise 3(a) a schematic diagram of a dark fieldscatterometer for use in measuring targets using a first pair ofillumination apertures, 3(b) a detail of diffraction spectrum of atarget grating for a given direction of illumination 3(c) a second pairof illumination apertures providing further illumination modes in usingthe scatterometer for diffraction based overlay measurements and 3(d) athird pair of illumination apertures combining the first and second pairof apertures;

FIG. 4 depicts a known form of multiple grating target and an outline ofa measurement spot on a substrate;

FIG. 5 depicts an image of the target of FIG. 4 obtained in thescatterometer of FIG. 3;

FIG. 6 is a flowchart showing the steps of an overlay measurement methodusing the scatterometer of FIG. 3 and adaptable to form embodiments ofthe present invention;

FIG. 7 illustrates some of the main diffraction modes resultant fromdiffraction by an overlay target in a known manner;

FIGS. 8(a)-8(c) comprise flowcharts of different aspects of an exemplarytarget design method usable in designing targets disclosed herein;

FIG. 9 is a perspective view showing trajectories of example raysthrough an exemplary target;

FIG. 10 is a perspective view of the arrangement of FIG. 9 from adifferent angle;

FIG. 11 is a perspective view of the arrangement of FIG. 9 from afurther different angle;

FIG. 12 is a side sectional view of the arrangement of FIG. 9;

FIG. 13 is a top view of an example second target structure in a target;

FIG. 14 is a top view of an example first target structure in the targetof FIG. 13;

FIG. 15 depicts detected fringe patterns formed from two pairs ofoverlapping target sub-structures;

FIG. 16 shows plots of signal intensity (vertical axis) against position(horizontal axis) for fringe patterns in a region of interest;

FIGS. 17(a) and 17(b) respectively depict a frequency spectrum and phasespectrum of the fringe patterns of FIG. 16;

FIG. 18 is a plot of measurements of phase for different wavelengths ofmeasurement radiation at two different overlay values;

FIG. 19 is a plot of measurements of phase for different targetthicknesses (layer thickness) at five different overlay values;

FIGS. 20(a) and 20(b) are top views respectively of a further examplesecond target structure and a further example first target structure;

FIG. 21 depicts trajectories of example rays through a target comprisinga pair of overlapping target sub-structures having first periodiccomponents with the same pitch;

FIG. 22 is a Fourier space representation of diffraction from theoverlapping target sub-structures shown in FIG. 21;

FIG. 23 depicts four intensity sub-regions resulting from scatteringfrom four differently biased pairs of overlapping target sub-structuresof the type depicted in FIG. 21;

FIG. 24 is a top view of four differently biased pairs of overlappingtarget sub-structures of the type depicted in FIG. 21;

FIG. 25 is a side sectional view perpendicular to plane X-X shown inFIG. 24;

FIG. 26 is a side sectional view perpendicular to plane Y-Y shown inFIG. 24;

FIG. 27 is a plot of expected intensity variation against overlayoffset, showing four expected intensity values corresponding to fourdifferently biased pairs of overlapping target sub-structures of thetype depicted in FIG. 21;

FIG. 28 depicts trajectories of example rays through a target comprisinga pair of overlapping target sub-structures having first periodiccomponents with different pitch;

FIG. 29 is a Fourier space representation of diffraction from theoverlapping target sub-structures shown in FIG. 28;

FIG. 30 is a top view of two pairs of overlapping target sub-structuresof the type depicted in FIG. 29;

FIG. 31 is a side sectional view perpendicular to plane X-X shown inFIG. 30;

FIG. 32 is a side sectional view of a target comprising four targetstructures with different pitches in different layers;

FIGS. 33(a)-33(c) illustrate a correspondence between a method ofmeasuring overlay error using pairs of target sub-structures with commonpitch and different overlay bias and a method of measuring overlay errorusing a pair of target sub-structures with different pitch.

FIG. 34 depicts a checkerboard pattern with rectangular elements;

FIG. 35 depicts a tilted checkerboard pattern;

FIGS. 36(a)-36(b), 37(a)-37(b), 38(a)-38(b) and 39(a)-39(b) depictfurther examples of pairs of target structures;

FIGS. 40(a)-(c) depict trajectories in 2D of example rays through atarget with double diffraction (a) in reflection then transmission (2waves), (b) in transmission then reflection (2 waves), and (c) in bothreflection/transmission then in transmission/reflection (4 waves);

FIGS. 41(a)-(c) depict trajectories 2D of coincident example raysthrough a target with double diffraction (a) in reflection thentransmission, (b) in transmission then reflection, and (c) in bothreflection/transmission then in transmission/reflection;

FIGS. 42(a)-(c) depict trajectories through an unfolded path in 2D ofcoincident example rays with double diffraction (a) in reflection thentransmission, (b) in transmission then reflection, and (c) in bothreflection/transmission then in transmission/reflection;

FIGS. 43(a)-(c) depict trajectories in 3D of coincident example raysthrough an unfolded path with double diffraction (a) in reflection thentransmission, (b) in transmission then reflection, and (c) in bothreflection/transmission then in transmission/reflection;

FIGS. 44(a)-(c) depict trajectories 2D of coincident example raysthrough a target with double diffraction (a) in reflection thentransmission, (b) in transmission then reflection, and (c) in bothreflection/transmission then in transmission/reflection;

FIGS. 45(a)-(c) depict Fourier space representations of doublediffraction (a) by a chequerboard then a grating, (b) by a grating thena chequerboard, and (c) by both a chequerboard/grating then by agrating/chequerboard;

FIG. 46 depicts trajectories through an unfolded path in 3D of examplerays coincident at an arbitrary angle with double diffraction in bothreflection/transmission then in transmission/reflection;

FIG. 47 depicts trajectories through an unfolded path in 2D of examplerays, coincident at an arbitrary angle in the xz plane, with doublediffraction in both reflection/transmission then intransmission/reflection;

FIG. 48 is a graph of the path lengths of different rays as a functionof incidence angle;

FIG. 49 depicts a Fourier space representation of double diffraction,with illumination at an arbitrary angle in the xz plane, by both achequerboard/grating then by a grating/chequerboard;

FIG. 50 depicts trajectories through an unfolded path in 3D of examplerays coincident at an illumination angle in the xz plane configured tolimit propagation of double diffraction in both reflection/transmissionthen in transmission/reflection;

FIG. 51 depicts trajectories through an unfolded path in 2D of examplerays, coincident at an illumination angle in the xz plane configured tolimit propagation of double diffraction in both reflection/transmissionthen in transmission/reflection;

FIG. 52 depicts a Fourier space representation of double diffraction,with an illumination angle in the xz plane configured to limitpropagation by both a chequerboard/grating then by agrating/chequerboard;

FIG. 53(a) depicts a Fourier space representation of double diffraction,with an illumination angle in the xz plane configured to limitpropagation by both a chequerboard/grating then by agrating/chequerboard to an offset detection aperture;

FIG. 53(b) depicts an illumination profile and offset detection aperturecorresponding to the Fourier space representation of FIG. 53(a);

FIG. 54(a) depicts a Fourier space representation of double diffraction,with an illumination angle in the xz plane configured to limitpropagation by both a chequerboard/grating then by agrating/chequerboard to a quadrant detection aperture;

FIG. 54(b) depicts an illumination profile and quadrant detectionaperture corresponding to the Fourier space representation of FIG.54(a);

FIG. 55(a) depicts a Fourier space representation of double diffraction,with an illumination angle in the xz plane configured to limitpropagation by both a chequerboard/grating then by agrating/chequerboard to a quad wedge; and

FIG. 55(b) depicts an illumination profile and quad wedge correspondingto the Fourier space representation of FIG. 55(a);

FIG. 56(a) depicts a Fourier space representation of double diffraction,with a quadrant illumination profile configured to limit propagation byboth a chequerboard/grating then by a grating/chequerboard to a quadwedge;

FIG. 56(b) depicts a quadrant illumination profile and quad wedgecorresponding to the Fourier space representation of FIG. 56(a);

FIG. 57(a) depicts a Fourier space representation of the firstdiffraction of the incident wave by the top grating, with a pointsymmetrized quadrant illumination profile;

FIG. 57(b) depicts a point symmetrized quadrant illumination profile andquad wedge corresponding to the Fourier space representation of FIG.57(a);

FIG. 58(a) depicts a Fourier space representation of the firstdiffraction of the incident wave by a stretched and 45-degree rotatedcheckerboard, with an offset illumination profile;

FIG. 58(b) depicts a stretched and 45-degree rotated checkerboard thatcauses diffraction depicted in the Fourier space representation of FIG.58(a);

FIG. 59(a) depicts a Fourier space representation of double diffractionvia a stretched checkerboard, with an illumination profile configured tolimit propagation by both the chequerboard/grating then by thegrating/chequerboard to a 45-degree rotated quad wedge;

FIG. 59(b) depicts a stretched checkerboard, rotated with respect to thequad wedge, that causes diffraction depicted in the Fourier spacerepresentation of FIG. 59(a);

FIG. 60(a), reproduced from FIG. 59(a), depicts a Fourier spacerepresentation of double diffraction via a stretched checkerboard, withan illumination profile configured to limit propagation by both thechequerboard/grating then by the grating/chequerboard to a 45-degreerotated quad wedge;

FIG. 60(b) depicts trajectories through an unfolded path in 2D of therays ending in quadrant Q2 in the Fourier space representation of FIG.60(a);

FIG. 61(a), like FIG. 59(a), depicts a Fourier space representation ofdouble diffraction via a stretched checkerboard, but with a pointsymmetrized illumination profile compared to FIG. 59(a);

FIG. 61(b) depicts trajectories through an unfolded path in 2D of therays ending in quadrant Q2 in the Fourier space representation of FIG.61(a);

FIG. 62(a), depicts a Fourier space representation of double diffractionvia a stretched checkerboard, combining the illumination profiles ofboth FIGS. 60(a) and 61(a); and

FIG. 62(b) depicts the combined trajectories of FIGS. 60(a) and 61(b)through an unfolded path in 2D of the rays ending in quadrant Q2 in theFourier space representations of FIG. 62(a).

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination optical system (illuminator) IL configured tocondition a radiation beam B (e.g., UV radiation or DUV radiation), apatterning device support or support structure (e.g., a mask table) MTconstructed to support a patterning device (e.g., a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters; a substratetable (e.g., a wafer table) WT constructed to hold a substrate (e.g., aresist coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate in accordance withcertain parameters; and a projection optical system (e.g., a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C(e.g., including one or more dies) of the substrate W.

The illumination optical system may include various types of opticalcomponents, such as refractive, reflective, magnetic, electromagnetic,electrostatic or other types of optical components, or any combinationthereof, for directing, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection optical system PS, which focuses the beam onto a targetportion C of the substrate W, thereby projecting an image of the patternon the target portion C. With the aid of the second positioner PW andposition sensor IF (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g., so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

The depicted apparatus can be used in a variety of modes, including forexample a step mode or a scan mode. The construction and operation oflithographic apparatus is well known to those skilled in the art andneed not be described further for an understanding of the presentinvention.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic system, referred to as a lithographic cell LC or alithocell or cluster. The lithographic cell LC may also includeapparatus to perform pre- and post-exposure processes on a substrate.Conventionally these include spin coaters SC to deposit resist layers,developers DE to develop exposed resist, chill plates CH and bake platesBK. A substrate handler, or robot, RO picks up substrates frominput/output ports I/O1, I/O2, moves them between the different processapparatus and delivers then to the loading bay LB of the lithographicapparatus. These devices, which are often collectively referred to asthe track, are under the control of a track control unit TCU which isitself controlled by the supervisory control system SCS, which alsocontrols the lithographic apparatus via lithography control unit LACU.Thus, the different apparatus can be operated to maximize throughput andprocessing efficiency.

A metrology apparatus is shown in FIG. 3(a). A target T and diffractedrays of measurement radiation used to illuminate the target areillustrated in more detail in FIG. 3(b). The metrology apparatusillustrated is of a type known as a dark field metrology apparatus. Themetrology apparatus may be a stand-alone device or incorporated ineither the lithographic apparatus LA, e.g., at the measurement station,or the lithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. In thisapparatus, light emitted by source 11 (e.g., a xenon lamp) is directedonto substrate W via a beam splitter 15 by an optical system comprisinglenses 12, 14 and objective lens 16. These lenses are arranged in adouble sequence of a 4F arrangement. A different lens arrangement can beused, provided that it still provides a substrate image onto a detector,and simultaneously allows for access of an intermediate pupil-plane forspatial-frequency filtering. Therefore, the angular range at which theradiation is incident on the substrate can be selected by defining aspatial intensity distribution in a plane that presents the spatialspectrum of the substrate plane, here referred to as a (conjugate) pupilplane. In particular, this can be done by inserting an aperture plate 13of suitable form between lenses 12 and 14, in a plane which is aback-projected image of the objective lens pupil plane. In the exampleillustrated, aperture plate 13 has different forms, labeled 13N and 13S,allowing different illumination modes to be selected. The illuminationsystem in the present examples forms an off-axis illumination mode. Inthe first illumination mode, aperture plate 13N provides off-axis from adirection designated, for the sake of description only, as ‘north’. In asecond illumination mode, aperture plate 13S is used to provide similarillumination, but from an opposite direction, labeled ‘south’. Othermodes of illumination are possible by using different apertures. Therest of the pupil plane is desirably dark as any unnecessary lightoutside the desired illumination mode will interfere with the desiredmeasurement signals.

As shown in FIG. 3(b), target T is placed with substrate W normal to theoptical axis O of objective lens 16. The substrate W may be supported bya support (not shown). A ray of measurement radiation I impinging ontarget T from an angle off the axis O gives rise to a zeroth order ray(solid line O) and two first order rays (dot-chain line +1 and doubledot-chain line −1). It should be remembered that with an overfilledsmall target, these rays are just one of many parallel rays covering thearea of the substrate including metrology target T and other features.Since the aperture in plate 13 has a finite width (necessary to admit auseful quantity of light, the incident rays I will in fact occupy arange of angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown. Note that the grating pitches of the targetsand the illumination angles can be designed or adjusted so that thefirst order rays entering the objective lens are closely aligned withthe central optical axis. The rays illustrated in FIGS. 3(a) and 3(b)are shown somewhat off axis purely to enable them to be more easilydistinguished in the diagram.

At least the 0 and +1 orders diffracted by the target T on substrate Ware collected by objective lens 16 and directed back through beamsplitter 15. Returning to FIG. 3(a), both the first and secondillumination modes are illustrated, by designating diametricallyopposite apertures labeled as north (N) and south (S). When the incidentray I of measurement radiation is from the north side of the opticalaxis, that is when the first illumination mode is applied using apertureplate 13N, the +1 diffracted rays, which are labeled +1(N), enter theobjective lens 16. In contrast, when the second illumination mode isapplied using aperture plate 13S the −1 diffracted rays (labeled −1(S))are the ones which enter the lens 16.

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements of thefirst order beam. The pupil plane image can also be used for manymeasurement purposes such as reconstruction.

In the second measurement branch, optical system 20, 22 forms an imageof the target T on sensor 23 (e.g. a CCD or CMOS sensor). In the secondmeasurement branch, an aperture stop 21 is provided in a plane that isconjugate to the pupil-plane. Aperture stop 21 functions to block thezeroth order diffracted beam so that the image of the target formed onsensor 23 is formed only from the −1 or +1 first order beam. The imagescaptured by sensors 19 and 23 are output to processor PU which processesthe image, the function of which will depend on the particular type ofmeasurements being performed. Note that the term ‘image’ is used here ina broad sense. An image of the grating lines as such will not be formed,if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and field stop 21 a shown inFIG. 3 are purely examples. On-axis illumination of the targets may beused and an aperture stop with an off-axis aperture used to passsubstantially only one first order of diffracted light to the sensor. Inyet other examples, 2nd, 3rd and higher order beams (not shown in FIG.3) can be used in measurements, instead of or in addition to the firstorder beams. In a further example, a pair of off-axis prisms 21 b areused in combination with an on-axis illumination mode. These prisms havethe effect of diverting the +1 and −1 orders to different locations onsensor 23 so that they can be detected and compared without the need fortwo sequential image capture steps. This technique, is disclosed in thepublished patent application US2011102753A1, the contents of which arehereby incorporated by reference. 2nd, 3rd and higher order beams (notshown in FIG. 3) can be used in measurements, instead of or in additionto the first order beams. U.S. Pat. No. 9,223,227B2, the contents ofwhich are hereby incorporated by reference, discloses a quadrature wedgethat redirects the light in the four quadrants of the pupil plane infour different directions the “quad wedge” is used with an “imagecopy-and-rotate” device that makes a copy of the illumination beam androtates the copied version over 180° relative to the original beam,providing a 180-degree point symmetry function that symmetrizes theillumination profile around the optical axis.

In order to make the measurement radiation adaptable to these differenttypes of measurement, the aperture plate 13 may comprise a number ofaperture patterns formed around a disc, which rotates to bring a desiredpattern into place. Note that aperture plate 13N or 13S can only be usedto measure gratings oriented in one direction (X or Y depending on theset-up). For measurement of an orthogonal grating, rotation of thetarget through 90° and 270° might be implemented. Different apertureplates are shown in FIGS. 3(c) and (d). The use of these, and numerousother variations and applications of the apparatus are described inprior published applications, mentioned above.

FIG. 4 depicts an overlay target or composite overlay target formed on asubstrate according to known practice. The overlay target in thisexample comprises four sub-overlay targets (e.g., gratings) 32 to 35positioned closely together so that they will all be within ameasurement spot 31 formed by the metrology radiation illumination beamof the metrology apparatus. The four sub-overlay targets thus are allsimultaneously illuminated and simultaneously imaged on sensors 19 and23. In an example dedicated to measurement of overlay, gratings 32 to 35are themselves composite gratings formed by overlying gratings that arepatterned in different layers of the semi-conductor device formed onsubstrate W. Gratings 32 to 35 may have differently biased overlayoffsets in order to facilitate measurement of overlay between the layersin which the different parts of the composite gratings are formed. Themeaning of overlay bias will be explained below with reference to FIG.7. Gratings 32 to 35 may also differ in their orientation, as shown, soas to diffract incoming radiation in X and Y directions. In one example,gratings 32 and 34 are X-direction gratings with offsets +d, −d,respectively. Gratings 33 and 35 are Y-direction gratings with offsets+d and −d respectively. Separate images of these gratings can beidentified in the image captured by sensor 23. This is only one exampleof an overlay target. An overlay target may comprise more or fewer than4 gratings, or only a single grating.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the overlay target of FIG. 4 in the apparatus ofFIG. 3, using the aperture plates 13NW or 13SE from FIG. 3(d). While thepupil plane image sensor 19 cannot resolve the different individualgratings 32 to 35, the image sensor 23 can do so. The dark rectanglerepresents the field of the image on the sensor, within which theilluminated spot 31 on the substrate is imaged into a correspondingcircular area 41. Within this, rectangular areas 42-45 represent theimages of the small overlay target gratings 32 to 35. If the overlaytargets are located in product areas, product features may also bevisible in the periphery of this image field. Image processor andcontroller PU processes these images using pattern recognition toidentify the separate images 42 to 45 of gratings 32 to 35. In this way,the images do not have to be aligned very precisely at a specificlocation within the sensor frame, which greatly improves throughput ofthe measuring apparatus as a whole.

Once the separate images of the overlay targets have been identified,the intensities of those individual images can be measured, e.g., byaveraging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan important example of such a parameter.

FIG. 6 illustrates how, using for example the method described inapplication WO 2011/012624, overlay error (i.e., undesired andunintentional overlay misalignment) between the two layers containingthe component overlay targets 32 to 35 is measured. Such a method may bereferred to as micro diffraction based overlay (μDBO). This measurementis done through overlay target asymmetry, as revealed by comparing theirintensities in the +1 order and −1 order dark field images (theintensities of other corresponding higher orders can be compared, e.g.+2 and −2 orders) to obtain a measure of the intensity asymmetry. Atstep S1, the substrate, for example a semiconductor wafer, is processedthrough a lithographic apparatus, such as the lithographic cell of FIG.2, one or more times, to create an overlay target including the gratings32-35. At S2, using the metrology apparatus of FIG. 3 for example, animage of the overlay targets 32 to 35 is obtained using only one of thefirst order diffracted beams (say −1). At step S3, whether by changingthe illumination mode, or changing the imaging mode, or by rotatingsubstrate W by 180° in the field of view of the metrology apparatus, asecond image of the overlay targets using the other first orderdiffracted beam (+1) can be obtained. Consequently the +1 diffractedradiation is captured in the second image.

Note that, by including only half of the first order diffractedradiation in each image, the ‘images’ referred to here are notconventional dark field microscopy images. The individual overlay targetlines of the overlay targets will not be resolved. Each overlay targetwill be represented simply by an area of a certain intensity level. Instep S4, a region of interest (ROI) is identified within the image ofeach component overlay target, from which intensity levels will bemeasured.

Having identified the ROI for each individual overlay target andmeasured its intensity, the asymmetry of the overlay target, and henceoverlay error, can then be determined. This is done (e.g., by theprocessor PU) in step S5 comparing the intensity values obtained for +1and −1 orders for each overlay target 32-35 to identify their intensityasymmetry, e.g., any difference in their intensity. The term“difference” is not intended to refer only to subtraction. Differencesmay be calculated in ratio form. In step S6 the measured intensityasymmetries for a number of overlay targets are used, together withknowledge of any known imposed overlay biases of those overlay targets,to calculate one or more performance parameters of the lithographicprocess in the vicinity of the overlay target T. In the applicationsdescribed herein, measurements using two or more different measurementrecipes will be included. A performance parameter of great interest isoverlay.

FIG. 7 illustrates a typical diffraction configuration of an overlaytarget comprising overlapping periodic structures. The overlappingperiodic structures comprise a first periodic structure (or firstgrating) and a second periodic structure (or second grating). In thespecific example shown, there is a first (lower) grating 700 in a firstlayer and a second (upper) grating 710 in a second layer, all formed ona substrate 705. Between the first grating 700 and second grating 710 islayer material 740, which (in this schematic example) may include thematerial that the second layer structures will be etched into.Measurement radiation 720 is incident on the second grating 710,resulting in diffraction forming non-zero (e.g., first) diffractionorders 730. In addition, some of the measurement radiation 720 (thezeroth order) passes through second grating 710 and layer material 740to be incident on the first grating 700, where again there isdiffraction forming non-zero (e.g., first) diffraction orders 750. Thenon-zero diffraction orders 730 from the second grating 710 and non-zerodiffraction orders 750 from the first grating 700 eventually interfere(e.g., in the far field) to form an overlay signal which can be capturedby a sensor (e.g., sensor 19 or sensor 23 of the apparatus depicted inFIG. 3(a)). Note that this diagram is provided only to illustrate therelevant principle of generating an overlay signal and, for simplicity,does not show all the diffraction modes (e.g., the transmissivediffraction modes are not shown). As has already been described, theremay be a deliberate offset (not shown) between the first grating 700 andsecond grating 710.

A metrology target design platform, such as D4C, may be used indesigning the metrology (overlay) targets. D4C enables a user to be ableto perform all required steps to design metrology targets withoutintervention from the creator of the D4C program. Appropriate graphicuser interfaces (GUI) are made available to set up, execute, review anduse the features of the D4C program. Usually, no special interface withthe fabrication tools is needed, because the metrology target design ismostly confined in the simulation domain rather than in the actualdevice manufacturing domain.

Conventional target design tools, such as multi-physics 3-D modelingsoftware, usually “draw” or “build” a geometric structure using area orvolume elements which are purely graphical. Those graphical elements areassigned multi-physics parametric characteristics. The fundamentaldifference of the D4C method with the conventional method is that thelithography process itself drives the rendering of the 3D structure ofthe metrology targets, so the designers do not have to build the modelelement-by-element.

FIG. 8(a) shows a flowchart that lists the main stages of the D4Cmethod. In stage 1110, the materials to be used in the lithographyprocess are selected. The materials may be selected from a materialslibrary interfaced with D4C through an appropriate GUI. In stage 1120, alithography process is defined by entering each of the process steps,and building a computer simulation model for the entire processsequence. In stage 1130, a metrology target is defined, i.e. dimensionsand other characteristics of various features included in the target areentered into the D4C program. For example, if a grating is included in astructure, then the number of grating elements, width of individualgrating elements, spacing between two grating elements etc. have to bedefined. In stage 1140, the 3D geometry is created. This step also takesinto account if there is any information relevant to a multi-layertarget design, for example, the relative shifts between differentlayers. This feature enables multi-layer target design. In stage 1150,the final geometry of the designed target is visualized. As will beexplained in greater detail below, not only the final design isvisualized, but as the designer applies various steps of the lithographyprocess, he/she can visualize how the 3D geometry is being formed andchanged because of process-induced effects. For example, the 3D geometryafter resist patterning is different from the 3D geometry after resistremoval and etching.

An important aspect of the present disclosure is that the targetdesigner is enabled to visualize the stages of the method to facilitatetheir perception and control during modeling and simulation. Differentvisualization tools, referred to as “viewers,” are built into the D4Csoftware. For example, as shown in FIG. 8(b), a designer can viewmaterial plots 1160 (and may also get a run time estimation plot)depending on the defined lithography process and target. Once thelithography model is created, the designer can view the model parametersthrough model viewer tool 1175. Design layout viewer tool 1180 may beused to view the design layout (e.g., visual rendering of the GDS file).Resist profile viewer tool 1185 may be used to view pattern profiles ina resist. Geometry viewer tool 1190 may be used to view 3D structures ona wafer. A pupil viewer tool 1195 may be used to view simulated responseon a metrology tool. Persons skilled in the art would understand thatthese viewing tools are available to enhance the understanding of thedesigner during design and simulation. One or more of these tools maynot be present in some embodiments of D4C software, and additionalviewing tools may be there in some other embodiments.

D4C enables designers to design thousands or even millions of designs.Not all of these designs will generate a required overlay signal. Todetermine one or a subset of such target designs which generate overlaysignals, the D4C method allows many designs to be evaluated andvisualized. Therefore it is possible to identify which targets generatethe required overly signals (and which of these provide the best overlayresponse, and/or are most robust to process variation etc.).

FIG. 8(c) shows a flow chart that illustrates how the D4C processincreases efficiency in the overall simulation process by reducing thenumber of metrology targets selected for the actual simulation of thelithography process. As mentioned before, D4C enables designers todesign thousands or even millions of designs. Not all of these designsmay be robust against variations in the process steps. To select asubset of target designs that can withstand process variation, alithographer may intentionally perturb one or more steps of the definedlithography process, as shown in block 1152. The introduction of theperturbation alters the entire process sequence with respect to how itwas originally defined. Therefore, applying the perturbed processsequence (block 1154) alters the 3D geometry of the designed target too.A lithographer only selects the perturbations that show nonzeroalternations in the original design targets and creates a subset ofselected process perturbations (block 1156). The lithography process isthen simulated with this subset of process perturbations (block 1158).Embodiments described below relate to a method of measurement of alithographic characteristic using a metrology target and a metrologyapparatus.

Optical metrology uses light scattered from a target to provideinformation about a lithographic process. The measurements are performedin optical instruments such as scatterometers. The information thatscatterometers are suitable to measure is, for example, overlay, whichis a relative distance between two overlapping gratings, in a planeparallel with the two overlapping gratings.

In a diffraction based overlay measurement, the overlay is extractedfrom a difference in the light intensity for the first positive andnegative first diffracted order. Examples of known scatterometerssuitable to measure overlay from diffracted light include thosescatterometers described in US2006033921A1, US2010201963A1,US2006066855A1, US2014192338, US2011069292A1, US20110027704A,US20110043791A, US2011102753A1, US20120044470A, US20120123581A,US20130258310A, US20130271740A, WO2016083076A1 and 62/320,780. Thecontents of all these applications are specifically and entirelyincorporated herein by reference.

Further, it is desirable to be able to use a metrology method whichprovides an optimal and robust result, which in turn leads to accurateoverlay measurement.

One of the problems faced by metrology applications for measuringoverlay is intensity perturbations that disrupt the balance inintensities diffracted from the two overlapping gratings. Further, incurrent measuring methods, there are few options to distinguish betweenintensity variations resulting from overlay and intensity variationsresulting from changes in thickness, or wavelength of scattered light.Another source of perturbations arises due to the finiteness of thetarget, which is manifested in strong signals, such as the edge effects.Furthermore, aberrations of the imaging optics are also a source ofintensity perturbations. In the currently known methods of measuringoverlay with scatterometry, the overlay sensitivity of the signal issensitive to layer thickness variations. This is also a challenge thatis solved with the embodiment disclosed in this patent application.

It is an object of embodiments disclosed herein to provide a method ofaccurate and robust measurement of a lithographic characteristic such asoverlay. Further, it is an object of embodiments disclosed herein toprovide a method of accurate and robust measurement of a lithographiccharacteristic such as overlay, wherein the measured overlay isindependent of the thickness of the stack, thickness such as thedistance between the two overlapping gratings. Furthermore, it is anobject of embodiments disclosed herein to provide a method of accurateand robust measurement of a lithographic characteristic such as overlay,wherein the measured overlay is independent of the wavelength of thelight used to illuminate the metrology target.

To address the above mentioned drawbacks, a target 60 comprising twooverlapping gratings is proposed, such as the target 60 of FIGS. 9-12.The target 60 in this example comprises a top grating (an example of asecond target structure 92 as referred to below) formed by lines with apitch P₂ and a chessboard (also referred to as checkerboard) grating (anexample of a first target structure 91 as referred to below) with apitch P₁ in a direction parallel with the pitch of top grating P₂ andpitch P_(H) in a direction perpendicular to the pitches P₂ and P₁. Whenilluminated with light in the visible or infrared or near infrared orultraviolet or EUV spectrum, the rays follow the paths as depicted inFIGS. 9-12. Normal incidence light (when viewed as in FIGS. 11 and 12)will be diffracted by the bottom grating (first target structure 91) andthe resulting diffracted orders +1 and −1 will be diffracted orscattered by the top grating (second target structure 92). The two rays(78A,78B) diffracted by the top grating, having an angle between them of2*θ₂, are interfering and forming a fringe pattern. The fringe patternwill be detected by a light intensity sensor, such as a camera orphotodiode (or plurality of photodiodes) and will form an image having aperiodic oscillating pattern. The fringe period, P_(f), is a functiononly of the pitches P₁ and P₂. The angles θ₁ and θ₂ and the period ofthe fringes P_(f) are given in below equations, wherein m is an integer.

$\begin{matrix}{\theta_{1} = {{asin}\left( \frac{m\;\lambda}{P_{1}} \right)}} & {{equation}\mspace{14mu} 1l} \\{\theta_{2} = {{asin}\left( {\frac{m\;\lambda}{P_{2}} - {\sin\;\theta_{1}}} \right)}} & {{equation}\mspace{14mu} 2} \\{P_{f} = {\frac{\lambda}{2\;\sin\;\theta_{2}} = \frac{P_{1}P_{2}}{2{{P_{2} - P_{1}}}}}} & {{equation}\mspace{14mu} 3}\end{matrix}$

One example of a preferred target is shown in FIGS. 13 and 14. FIG. 13depicts an example top grating. FIG. 14 depicts an example bottomgrating. The fringe pattern from the target of FIGS. 13 and 14 is shownin FIG. 15. The overlay (the relative shift between the top and bottomgrating) is extracted from the shift in the periodic fringe pattern.With known signal processing techniques, the phase φ of the periodicsignal is extracted, the periodic signal having a frequency equal to thefringe frequency, determined by P₁ and P₂, as shown in equation 3. Stepsin an example signal processing procedure are shown in FIGS. 16 and 17.FIG. 16 shows plots of signal intensity (vertical axis) against position(horizontal axis) for fringe patterns in a region of interest 80 arisingfrom two adjacent pairs of overlapping gratings. A shift in the periodicfringe pattern due to overlay error provides a corresponding phase shiftφ. The phase shift φ can be determined from a spatial Fourier analysisof the fringe patterns as shown in FIG. 17. FIG. 17(a) depicts afrequency spectrum of the fringe pattern. FIG. 17(b) depicts a phasespectrum of the fringe pattern. The peak in the frequency spectrum isdetermined by the fringe period P_(f). The phase corresponding to thepeak (indicated by broken line arrow) provides information about theshift in the fringe pattern, and therefore the overlay error.

A target 60 with a chessboard pattern as a top grating and a line/spacepattern as a bottom grating works on a similar principle.

FIGS. 18 and 19 show the dependence of a parameter, such as phase of theperiodic signal having a frequency given by the fringe pattern, as afunction of wavelength used to illuminate the target 60 (in FIG. 18) andas a function of the stack thickness (in FIG. 19). The overlay, which isdirectly proportional to the phase according to equation 4, is notdependent on wavelength or stack thickness.

$\begin{matrix}{{Overlay} = {\frac{P\; 1P\; 2}{4{\pi\left( {{P\; 1} + {P\; 2}} \right)}} \cdot \varphi}} & {{equation}\mspace{14mu} 4}\end{matrix}$

An alternative design according to an embodiment is shown in FIG. 20, inwhich FIG. 20(a) depicts a top grating and FIG. 20(b) depicts a bottomgrating. A single target is provided with two-dimensional gratings onboth top and bottom layers. The top target consists of a 2D grating withpitch P₁ (e.g., 500 nm) in both X and Y directions. The bottom target isa checkerboard with pitch P₂ (e.g., 450 nm) in both X and Y. In thiscase P_(H)=P₂. Fiducials consisting of interference fringes wouldsurround the target shown, or placed adjacent, using reversed P₁ and P₂on top and bottom. Fixed, periodically-segmented diffraction gratingscan also be used for the phase fiducials.

The illumination direction would determine which overlay sensitivity isdisplayed currently: for X overlay measurement the target would beilluminated predominantly from the Y direction; for Y overlaymeasurement the target would be illuminated predominantly from the Xdirection. A different set of interference fringes would be produceddepending on the illumination direction and the XY overlay condition.

An advantage to this target design is that it reduces target footprintarea by not having duplicated X and Y targets. Another advantage is thatthe full grating area can be used for measurement in a particulardirection, i.e, the full area will be covered by interference fringes,enhancing design flexibility, overlay sensitivity due to larger fringedisplacement magnification, and noise reduction by averaging over alarger set of fringes.

An advantage of the current invention is measurement of overlay signalindependent of the wavelength and stack layer thickness. This advantageis based on equal path lengths of the +1 and −1 diffracted orders.Further advantage of the invention is insensitivity to edge effectssince the phase shift is orthogonal to the target edge facing theillumination source.

FIGS. 21-27 depict yet another embodiment. FIG. 21 shows a target 60having gratings with equal pitches P illuminated with light (measurementradiation 72). A specific example of such a target 60 is depicted inFIGS. 24-26 and described in detail further below. The image of such atarget 60, when illuminated with light in a diffraction basedscatterometer, may resemble the squares labeled 141-144 in FIG. 23(which may be referred to as intensity sub-regions). The squares 141-144are obtained when illuminating targets having a relative shift (overlaybias) between the top and bottom gratings with, for example, −P/8±d,P/8±d, where P is the equal pitch of the top and bottom grating and d isan arbitrary bias (predetermined constant). Such targets 60 may forexample comprise plural pairs 61-64 of overlapping target sub-structures151-158. Overlay can be extracted from a value proportional to theintensity in each of the images 141-144 or a relation thereof. Theintensity of the compound signal 78 in FIG. 23 is given by expressionssuch as in equations 5 (described in further detail below).

$\begin{matrix}{\mspace{76mu}{{I_{0,1} = {{{{A_{- 1}B_{1,1}} + {A_{1}B_{{- 1},1}}}}^{2} = {2A^{2}{B^{2}\left\lbrack {1 + {\cos\left( {4\pi\frac{X_{s}}{P}} \right)}} \right\rbrack}}}}{I_{0,{- 1}} = {{{{A_{- 1}B_{1,{- 1}}} + {A_{1}B_{{- 1},{- 1}}}}}^{2} = {2A^{2}{B^{2}\left\lbrack {1 + {\cos\left( {4\pi\frac{X_{s}}{P}} \right)}} \right\rbrack}}}}}} & {{equations}\mspace{14mu} 5}\end{matrix}$

The intensity of the component signal 78 varies as a function of theoverlay difference X_(s) between the top and bottom gratings as depictedin FIG. 27. Each of the four intensities shown I_(A-D) corresponds to adifferent one of the four overlay biases mentioned above (−P/8±d, P/8±d)with zero overlay error. An overlay error will additionally shift thecurve to the right or left. The shift can be calculated from the changein the intensities I_(A-D).

In a Fourier analysis of the diffracted rays of light, such as for theexample given in FIG. 22 (described in further detail below),interfering light between the first positive and negative diffractedorders will be independent of used wavelength and stack thickness, suchas the distance between the gratings. Using different illuminationapertures is a method to control the position of the various Fouriercomponents of the diffracted pattern, and therefore increasing thedetectability of the signal based only on the diffracted positive andnegative order, since such orders are highly sensitive to overlay.

According to an embodiment, a method of measuring a target 60 isprovided. The target is formed by a lithographic process. Specificexamples of such a method have been discussed above with reference toFIGS. 9-27. The method, and variations on the method, will be describedin further detail below.

As depicted in FIG. 12 for example, the target 60 comprises a layeredstructure. A first target structure 91 (a periodic structure) isprovided in a first layer 81. A second target structure 92 (a periodicstructure) is provided in a second layer 82. In between the first targetstructure 91 and the second target structure 92 is a layer material 85.The layer material 85 may (or may not) contain material that structuresforming the second target structure 92 are etched into. The layeredstructure is formed on a substrate 87.

The target 60 is illuminated with measurement radiation 72. The methodcomprises detecting scattered radiation formed by interference betweenplural predetermined (different) diffraction orders 74A,74B. Acharacteristic of the lithographic process used to form the target, suchas overlay error, is calculated using the detected scattered radiation.

The interfering predetermined diffraction orders 74A,74B are generatedby diffraction of the measurement radiation 72 from the first targetstructure 91. In an embodiment the interfering predetermined diffractionorders 74A,74B comprise or consist of two equal and opposite diffractionorders. In the particular example of FIG. 12, the interferingpredetermined diffraction orders comprise a −1 diffraction order(negative first order) and a +1 diffraction order (positive first order)(i.e. equal and opposite first diffraction orders). In otherembodiments, other predetermined diffraction orders may contribute tothe detected scattered radiation formed by interference (e.g. zerothorder or higher orders).

The interfering predetermined diffraction orders 74A,74B initiallydiverge from the target structure 91 at a relatively large angle θ1.Subsequent diffraction by the second target structure 92 causes theinterfering predetermined diffraction orders 74A,74B to be broughtcloser together (as depicted by rays 78A and 78B, which diverge at amuch smaller angle θ₂). Ray 78A is a −1 diffraction order generated fromray 74A and ray 78B is a +1 diffraction order generated from ray 74B.The method uses rays 78A and 78B to form an interference pattern (orregion of uniform intensity caused by interference) and uses theinterference pattern (or region of uniform intensity caused byinterference) to measure an overlay error. The detection of theinterference between the predetermined diffraction orders is enabled bythe subsequent diffraction from the second target structure 92. Thissubsequent diffraction brings the predetermined diffraction orders closeenough together for them to be received efficiently and simultaneouslyby an objective lens 16 of a detection system and for the interferencepattern (or region of uniform intensity caused by interference) to besensitive to overlay error. Where a metrology apparatus of the typeshown in FIG. 3(a) is used, for example, the interference pattern may bemeasured by the second measurement branch.

The path lengths of the interfering predetermined diffraction ordersthrough the layered structure are equal. Intensity variations in thedetected scattered radiation caused by path length effects (e.g. greaterattenuation for longer path lengths) are thus avoided. The measurementwill also be independent of the thickness of the target 60 (e.g. theseparation between the first target structure 91 and the second targetstructure 92, which may also be referred to as the thickness of thestack). The measurement will be effective for thin targets 60 and thicktargets 60. As will be described in detail below, the detectedinterference pattern (or region of uniform intensity caused byinterference) is also independent of the wavelength of the radiation.This reduces or avoids errors caused by changes or differences in thespectrum of measurement radiation as it propagates through the target60.

The detected scattered radiation formed by interference between thepredetermined diffraction orders varies as a function of overlay errorbetween the first target structure 91 and the second target structure92. There are multiple ways in which the first target structure 91 andsecond target structure 92 may be configured to provide the variation inthe detected scattered radiation as a function of overlay error.Specific examples have been described above. Further specific exampleswill be described below.

In an embodiment, the interfering predetermined diffraction orders74A,74B are generated by diffraction in reflection from the first targetstructure 91. The subsequent diffraction of the generated diffractionorders 74A, 74B from the second target structure 92 comprisesdiffraction in transmission through the second target structure 92. FIG.12 shows an embodiment of this type. In other embodiments, alternativegeometries may be used. For example, in other embodiments thepredetermined diffraction orders 74A, 74B are generated by diffractionin transmission through the first target structure 91. Additionally oralternatively, the subsequent diffraction comprises diffraction inreflection from the second target structure 92.

In an embodiment, the interfering predetermined diffraction orders aregenerated by diffraction in transmission through the first targetstructure and the subsequent diffraction of the predetermineddiffraction orders from the second target structure comprisesdiffraction in reflection from the second target structure. Such anembodiment could be used to provide rays 78A and 78B similar to thoseshown in FIG. 12, but the first target structure would in this case needto be above the second target structure instead of below. Thus, in suchan embodiment, the target structure labelled 91 in FIG. 12 wouldcorrespond to the second target structure and the target structurelabelled 92 would correspond to the first target structure. Measurementradiation 72 would diffract firstly from the target structure labelled92 (rather than passing straight through as in FIG. 12) and thendiffract a second time (in reflection) from the target structurelabelled 91 (before passing straight through the target structurelabelled 92 as rays 78A and 78B).

Thus, the measurement radiation 72 doubly diffracts in either order froman upper target structure and a lower target structure. The doublediffraction brings together different diffraction orders to producecharacteristic interference (with characteristic intensity and/or with afringe pattern of characteristic frequency and phase) that isinsensitive to wavelength and stack thickness. In practice, the twosequences of diffractions occur simulataneously and reinforce.Diffracted orders that originate from a single diffraction, or fromthree diffractions, are at significantly different angles and do notcontribute to the observed interference (e.g. intensity and/or fringepattern).

In an embodiment, the target 60 comprises three or more pairs 61-64 ofoverlapping target sub-structures 151-158. An example of such a target60 was mentioned briefly above with reference to FIGS. 21-27. Thestructure of an example target 60 is depicted in FIGS. 24-26. Each pair61-64 of overlapping target sub-structures 151-158 in such an embodimentcomprises a first target sub-structure 151-154. The first targetsub-structure 151-154 is provided in the first target structure 91 (i.e.in the first layer 81). Each pair 61-64 of overlapping targetsub-structures 151-158 in such an embodiment further comprises a secondtarget sub-structure 155-158. The second target sub-structure 155-158 isprovided in the second target structure 92 (i.e. in the second layer82). In the example of FIGS. 24-26, four pairs are provided. The firstpair 61 comprises a first target sub-structure 151 and a second targetsub-structure 155. The second pair 62 comprises a first targetsub-structure 152 and a second target sub-structure 156. The third pair63 comprises a first target sub-structure 153 and a second targetsub-structure 157. The fourth pair 64 comprises a first targetsub-structure 154 and a second target sub-structure 158.

Each of the first target sub-structure 151-154 and the second targetsub-structure 155-158 in each pair 61-64 of overlapping targetsub-structures 151-158 comprises a first periodic component (e.g. a linegrating or a periodic component of a checkerboard pattern) having thesame pitch and orientation. In the embodiment of FIGS. 21-27, the firstperiodic component comprises a grating having a pitch P along at leastone direction (e.g. a line grating with pitch P or a checkerboardpattern with pitch P). Each pair 61-64 of overlapping targetsub-structures 151-158 is provided with a different overlay bias.Providing the pairs 61-64 of overlapping target sub-structures 151-158with different overlay biases makes it possible for an overlay error tobe obtained with high reliability and/or high accuracy, as describedbelow.

FIG. 22 is a Fourier space representation of diffraction from the pair61 of overlapping target sub-structures 151, 155 shown in FIG. 21. Theskilled person would appreciate that the same principle would apply toeach of the other pairs 62-64 of overlapping target sub-structures152-154,156-158.

Graph 101 represents an expected diffraction pattern (Fourier transform)of the target sub-structure 155. Graph 102 represents an expecteddiffraction pattern (Fourier transform) of the target sub-structure 151.Graph 103 represents an expected diffraction pattern (Fourier transform)resulting from diffraction from a combined structure formed from acombination of the target sub-structure 155 and target sub-structure151. The combined structure is formed by superposing (or multiplying)the target sub-structure 155 with the target sub-structure 151. Thediffraction pattern of graph 103 can therefore be obtained byconvolution of the diffraction pattern of graph 101 with the diffractionpattern of graph 102, as shown in the FIG. 22.

The diffraction pattern of graph 101 comprises a −1, zeroth, and a +1diffraction order, represented respectively by localized peaks havingassociated Fourier coefficients A⁻¹, A₀ and A₁. All of the peaks arealigned along the horizontal axis because the target sub-structure 155in this example consists of a simple line grating (the first periodiccomponent). The only spatial periodicity is therefore represented by thepitch of the line grating, which is in turn represented by theseparation along the horizontal axis of the peaks A⁻¹ and A₁ (equal to2*k, where k=2π/P).

The diffraction pattern of graph 102 comprises more peaks because thetarget sub-structure 151 comprises both a first periodic component and asecond periodic component. The first periodic component is parallel tothe line grating in the target sub-structure 155 and has the same pitchP. The second periodic component is perpendicular to the first periodiccomponent (e.g. a checkerboard pattern). Peaks resulting fromdiffraction from the combination of the first periodic component and thesecond periodic component comprise a zeroth order peak with Fouriercoefficient B_(0,0) and first order peaks with Fourier coefficientsB_(−1,1), B_(1,1), B_(1,−1) and B_(−1,−1). The separation between peaksB_(−1,1) and B_(1,1) and between peaks B_(1,−1) and B_(−1,−1) isdetermined by the pitch P of the first periodic component. Theseparation between peaks B_(−1,1) and B_(−1,−1) and between B_(1,1) andB_(1,−1) is determined by the pitch of the second periodic component,which in the particular example shown is also P but could be any othervalue.

The convolution of the diffraction patterns of graphs 101 and 102effectively superposes the three peaks A⁻¹, A₀ and A₁ of graph 101 ateach of the positions of the peaks B_(0,0), B_(−1,1), B_(1,1), B_(1,−1)and B_(−1,−1) of graph 102. Due to the identical pitch P of the firstperiodic component in each of the two target sub-structures 151 and 155,first order diffraction peaks in the diffraction pattern of graph 103are formed from overlapping peaks generated from different predetermineddiffraction orders. The localized nature of the overlapping peaks inFourier space indicates that scattered radiation formed by interferencebetween the predetermined diffraction orders will be output from thetarget 60 at nominally the same angle (or within a small range ofangles). Each of the first order peaks along the vertical axis in graph103 is formed from overlap of a peak corresponding to +1 diffractionfrom the target sub-structure 151 followed by −1 diffraction from thetarget sub-structure 155 (A₁B_(−1,1) or A₁B_(−1,−1)) with a peakcorresponding to −1 diffraction from the target sub-structure 151followed by +1 diffraction from the target sub-structure 155 (A⁻¹B_(1,1)or A⁻¹B_(1,−1)). The predetermined diffraction orders that interferewith each other are thus defined with respect to diffraction from thefirst periodic component in the pair 61 (thus, in the present case thepredetermined diffraction orders are the +1 and −1 diffraction orderswith respect to the first periodic component having the period P in eachof the two target sub-structures 151 and 155). The intensities I_(0,1)and I_(0,−1) of scattered radiation produced by the overlapping peaks ingraph 103 are given by the expressions provided above in equations 5. Itis well known in metrology how to selectively measure scatteredradiation corresponding to selected regions in Fourier space, forexample by selecting a suitable illumination mode using an apertureplate 13 as described above with reference to FIG. 3. The intensitiesI_(0,1) and I_(0,−1) can therefore be measured.

Each of the intensities I_(0,1) and I_(0,−1) will vary as a function ofthe overlay offset between the first periodic components in theoverlapping target sub-structures 151-158 of each pair 61-64. An examplevariation of intensity with overlay offset is depicted in FIG. 27 andmentioned above. The variation is expected to be at least approximatelysinusoidal with an average intensity of I₀. In an embodiment, an overlayerror is detected by measuring a change in the position of the curve tothe right or to the left by measuring intensities at different overlaybiases.

In an embodiment, the different overlay biases comprise one or morepairs of equal and opposite overlay biases. Such overlay biases providea symmetric sampling of the intensity variation as a function of overlayoffset, which is expected to be less sensitive to the presence of higherharmonics in the signal. In an embodiment, the different overlay biasescomprise the following four biases: −P/8−d, P/8+d, −P/8+d and P/8−d,where P is the pitch of the first periodic component and d is apredetermined constant. An example of this type has been discussed abovewith reference to FIGS. 21-27. As can be seen from FIG. 27, theintensities I_(A-D) resulting from these four overlay biases arenominally symmetrically distributed about the origin. Additionally, bypositioning the overlay biases within a limited range either side of thesteepest part of the nominal curve (at P/8), all of the intensitiesI_(A-D) will be nominally positioned at regions having a relatively highsteepness. Any change in the position of the curve due to overlay errorwill therefore result in a relatively rapid change in the measuredintensities I_(A-D), thereby favoring high sensitivity.

The four intensities I_(A-D) are related to overlay error OV, in thecase where the biases are given by −P/8−d, P/8+d, −P/8+d and P/8−d, bythe following equations:I _(A) =I ₀ +K(OV−d)I _(B) =I ₀ +K(OV+d)I _(C) =I ₀ −K(OV−d)I _(D) =I ₀ −K(OV+d)

These four equations contain only three unknowns and so can be solved tofind OV:

${OV} = {d\left\lbrack \frac{\left( {I_{A} - I_{C}} \right) + \left( {I_{B} - I_{D}} \right)}{\left( {I_{A} - I_{C}} \right) - \left( {I_{B} - I_{D}} \right)} \right\rbrack}$

In embodiments of the type discussed above with reference to FIGS.21-27, the detected scattered radiation formed by interference betweenthe predetermined diffraction orders comprises a plurality of intensitysub-regions 141-144 (as shown in FIG. 23). Each intensity sub-region141-144 is formed by measurement radiation diffracted from a differentrespective pair of the three or more pairs 61-64 of targetsub-structures 151-158. In the particular example of FIG. 23, foursquare intensity sub-regions 141-144 are provided by the square arraytarget 60 depicted in FIGS. 24-26. Although each intensity sub-region141-144 is formed by interference, only a single value of intensity isobtained at any given time. The intensity sub-regions 141-144 do notindividually comprise an interference pattern having any interferencegenerated spatial structure (e.g. an interference fringe pattern). Thislack of spatial structure is a result of the high degree of overlap inFourier space of the peaks that are interfering to produce the detectedintensity. Detecting a single absolute value of a spatially uniformintensity rather than detecting a pattern having spatial structure isdesirable because existing methods of measuring overlay (as describedabove with reference to FIGS. 3-6) also rely on measurements of a singleabsolute value of intensity and can therefore be adapted particularlyefficiently to perform the present method.

In an alternative embodiment, an interference pattern is formed whichdoes have spatial structure and the spatial structure is used to extractoverlay. An example of such a method is described below with referenceto FIGS. 28-31.

In such a method, a target 60 is provided that comprises at least onepair 61, 62 of overlapping target sub-structures 151-154. An example ofsuch a target 60 is depicted in FIGS. 30 and 31. Each pair 61, 62 ofoverlapping target sub-structures 151-154 comprises a first targetsub-structure 151, 152 in the first target structure 91 (i.e. in thefirst layer 81) and a second target sub-structure 153, 154 in the secondtarget structure 92 (i.e. in the second layer 82). The first targetsub-structure 151, 152 and the second target sub-structure 153, 154 ineach pair 61, 62 of overlapping target sub-structures 151-154 comprisesa first periodic component having the same orientation and a differentpitch P₁, P₂. As described below, the different pitches P₁, P₂ providean interference pattern between different predetermined diffractionorders that has a spatial structure. In an embodiment, the detectedscattered radiation formed by interference between the predetermineddiffraction orders comprises a fringe pattern formed by each pair 61, 62of target sub-structures 151-154. In an embodiment, the fringe patternis such that a variation in overlay error between the first targetstructure 91 and the second target structure 92 causes a positionalshift of fringes in (i.e. a change in phase of) each fringe pattern.Overlay error can thus be obtained by extracting the phase of the fringepattern.

FIG. 29 is a Fourier space representation of diffraction from the pair61 of overlapping target sub-structures 151, 153 shown in FIG. 28. Theskilled person would appreciate that the same principle would apply tothe other pair 62 of overlapping target sub-structures 152, 154.

The pair 61 of overlapping target sub-structures 151, 153 shown in FIG.28 is the same as the pair 61 of overlapping target sub-structures 151,155 shown in FIG. 21 except that the pitch P₁ of the lower targetsub-structure 151 is different from the pitch P₂ of the upper targetsub-structure 153. Additionally, the target sub-structure 151 comprisesa second periodic component have a pitch P₃ (which may be equal to P₁ orP₂ or any other value).

Graph 201 represents an expected diffraction pattern (Fourier transform)of the target sub-structure 153. Graph 202 represents an expecteddiffraction pattern (Fourier transform) of the target sub-structure 151.Graph 203 represents an expected diffraction pattern (Fourier transform)resulting from diffraction from a combined structure formed from acombination of the target sub-structure 153 and target sub-structure151. The combined structure is formed by superposing (or multiplying)the target sub-structure 153 with the target sub-structure 151. Thediffraction pattern of graph 203 can therefore be obtained byconvolution of the diffraction pattern of graph 201 with the diffractionpattern of graph 202, as shown in the FIG. 29.

The diffraction pattern of graph 201 is the same as the diffractionpattern of graph 101 in FIG. 22 except that the separation between the−1 and +1 peaks is given by 2*k₂, where k₂=2π/P₂.

The diffraction pattern of graph 202 is the same as the diffractionpattern 102 in FIG. 22 except that the separation between the −1 and +1peaks along the horizontal direction is given by 2*k₁, where k₁=2π/P₁,and the separation between the −1 and +1 peaks along the verticaldirection is given by 2*k₃, where k₁=2π/P₃.

The convolution of the diffraction patterns of graphs 201 and 202effectively superposes the three peaks A⁻¹, A₀ and A₁ of graph 201 ateach of the positions of the peaks B_(0,0), B_(−1,1), B_(1,1), B_(1,−1)and B_(−1,−1) of graph 202. Due to the different pitches P₁ and P₂ ofthe first periodic component in each of the two target sub-structures151 and 153, a diffraction pattern is formed which comprises twodistinct peaks in the region of the overlapping first order peaks(corresponding to the predetermined diffraction orders) in graph 103 ofFIG. 22. The distinct peaks are separated from each other, as indicated,by 2π/P₂−2π/P₁. The peaks from these predetermined diffraction ordersare located close to each other in Fourier space and can therefore beextracted efficiently and used to form an intensity pattern in which thepredetermined diffraction orders interfere with each other. Each of thepairs of peaks from the predetermined diffraction orders in graph 203comprises a peak corresponding to +1 diffraction from the targetsub-structure 151 followed by −1 diffraction from the targetsub-structure 153 (A₁B_(−1,1) or A₁B_(−1,−1)) and a peak correspondingto −1 diffraction from the target sub-structure 151 followed by +1diffraction from the target sub-structure 153 (A⁻¹B_(1,1) orA⁻¹B_(−1,1)). The predetermined diffraction orders that interfere witheach other are thus defined with respect to diffraction from the firstperiodic component in the pair 61 (in this case the +1 and −1diffraction orders with respect to diffraction from the first periodiccomponent in the pair 61). The intensities I_(0,1) and I_(0,−1) of eachof these pairs of peaks in graph 203 are given by a generalized form ofthe equations 5, which are labelled equations 6 and given as follows:

$\begin{matrix}{{I_{0,1} = {{{{A_{- 1}B_{1,1}} + {A_{1}B_{{- 1},1}}}}^{2} = {2A^{2}{B^{2}\left\lbrack {1 + {\cos\left( {{4\pi\frac{X_{s}}{P_{2}}} + {4{\pi\left( {\frac{x}{P_{2}} - \frac{x}{P_{1}}} \right)}}} \right)}} \right\rbrack}}}}{I_{0,{- 1}} = {{{{A_{- 1}B_{1,{- 1}}} + {A_{1}B_{{- 1},{- 1}}}}}^{2} = {2A^{2}{B^{2}\left\lbrack {1 + {\cos\left( {{4\pi\frac{X_{s}}{P_{2}}} + {4{\pi\left( {\frac{x}{P_{2}} - \frac{x}{P_{1}}} \right)}}} \right)}} \right\rbrack}}}}} & {{equations}\mspace{14mu} 6}\end{matrix}$

Equations 6 differ from equations 5 in that the intensities I_(0,1) andI_(0,−1) further comprise a spatially periodic term having a pitch(which may also be referred to as a Moiré period) proportional to ½

${\frac{P_{1} - P_{2}}{P_{1}P_{2}}},$and a phase proportional to the overlay offset X_(s). The spatiallyperiodic term defines the pitch and phase of a fringe pattern formed bythe intensities I_(0,1) and I_(0,−1). Overlay error will cause a shiftin the phase of the fringe pattern. Measurement of the phase cantherefore be used to measure overlay error. A sensitivity of the phaseto overlay error can varied as desired by appropriate selection of P₁and P₂.

Producing an interference pattern having spatial structure (e.g. afringe pattern) enables filtering in the spatial frequency domain (asdepicted in FIG. 17). Contributions to detected radiation intensity thatare not relevant to overlay can be removed. Examples of intensitycontributors that can be excluded include target edge peaks, asymmetricillumination and scattered light from adjacent devices or otherstructures. The phase of the interference pattern can therefore beextracted with high accuracy and reliability.

The phase of the fringe pattern advantageously varies linearly withoverlay error throughout the +π to −π phase range. This linear variationfacilitates calibration and provides uniform sensitivity.

In an embodiment, an example of which is depicted in FIG. 32, the target60 further comprises a reference structure R1 to act as a phasereference. The reference structure R1 provides a radiation patternhaving the same periodicity as the fringe pattern. The referencestructure R1 is provided in such a way that there is substantially nopositional shift of fringes in the radiation pattern from the referencestructure R1 as a function of overlay error between the first targetstructure 91 and the second target structure 92. For example, thereference structure R1 may be formed entirely within a single layer ofthe target. Thus, a relative shift between the fringe pattern and theradiation pattern from the reference structure R1 can be used to obtainoverlay error. In the particular example of FIG. 32, the target 60comprises four target structures 91-94, but it will be appreciated thatthe principle can be applied to a target 60 contained only two targetstructures (as in FIGS. 30 and 31 for example) or any other number oftarget structures.

An alternative or additional approach to providing a phase reference isto provide a target 60 which produces fringes that move in oppositedirections relative to each other as a function of overlay error. Anexample target 60 of this type is depicted in FIGS. 30 and 31. Thetarget 60 comprises at least a first pair 61 of overlapping targetsub-structures 151 and 153, and a second pair 62 of overlapping targetsub-structures 152 and 154. In the first pair 61 of overlapping targetsub-structures 151 and 153, the first periodic component of the firsttarget sub-structure 151 has a first pitch P₁ and the first periodiccomponent of the second target substructure 153 has a second pitch P₂.In the second pair 62 of overlapping target sub-structures 152 and 154,the first periodic component of the first target sub-structure 152 hasthe second pitch P₂ and the first periodic component of the secondtarget substructure 154 has the first pitch P₁. It can be seen frominspection of equations 6 that swapping P₁ and P₂ in this manner resultsin the target 60 producing a fringe pattern from the first pair 61 ofoverlapping target sub-structures 151 and 153 that moves in an oppositedirection to a fringe pattern from the second pair 62 of overlappingtarget sub-structures 152 and 154, as a function of overlay error. FIGS.13 and 14 depict example patterns for a target 60 of this type. FIG. 13depicts patterns suitable for target sub-structures 153 (right) and 154(left). FIG. 14 depicts patterns suitable for target sub-structures 151(right) and 152 (left). FIG. 15 depicts example fringe patterns. Theright-hand fringe pattern corresponds to the first pair 161 ofoverlapping target sub-structures 151 and 153. The left-hand fringepattern corresponds to the second pair 62 of overlapping targetsub-structures 152 and 154.

FIG. 33 illustrates a correspondence between a method of measuringoverlay error using pairs of target sub-structures with common pitch anddifferent overlay bias (as described above for example with reference toFIGS. 21-27), and a method of measuring overlay error using a pair oftarget sub-structures with different pitch (as described above forexample with reference to FIGS. 28-32). FIG. 33(a) shows a portion of anexample first target structure 91 having pitch P₁ and an example secondtarget structure 92 having pitch P₂. The difference in pitch can beexploited to produce a fringe pattern as described above. Overlay errorcan be extracted from a change in the spatial phase of the fringepattern. The alternative approach of using multiple pairs of targetsub-structures with the same pitch P but different overlay biases can beviewed as effectively sampling the fringe pattern that would be producedusing target structures with different pitch. FIG. 33(b) shows examplesegments 401-405 extracted from the arrangement of FIG. 33(a). Eachsegment 401-405 will have a different average shift between the portionof the first target structure 91 and second target structure 92 in thesegment. Each segment 401-405 could then be approximated by a respectivepair of target sub-structures 411-415 which have the same pitch and anoverlay bias equal to the average shift of the segment 401-405, as shownin FIG. 33(c). Thus, for example, pair 411 has an overlay bias equal tothe average shift in segment 401, pair 412 has an overlay bias equal tothe average shift in segment 402, etc. The resulting multiple pairs ofsub-structures 411-415 of FIG. 33(c) thus provide an approximation ofthe full arrangement of FIG. 33(a). Measurement of the intensity valuescorresponding to the multiple pairs of target sub-structures 411-415 ofFIG. 33(c) effectively samples the variation of intensity of the fringepattern formed directly by the arrangement of FIG. 33(a). A shift in thephase of the fringe pattern can thus be detected. Overlay errorproportional to the shift in phase can therefore also be detected.

In an embodiment, as depicted for example in FIG. 32, the target 60comprises one or more further target structures 93, 94, respectively inone or more further layers 83, 84 of the layered structure. The furthertarget structures 93, 94 are thus provided in addition to the firsttarget structure 91 and second target structures 92 (respectively inlayers 81 and 82) of the embodiment discussed above. In such anembodiment, the target 60 comprises at least one pair of overlappingtarget sub-structures in each of a plurality of different respectivepairs of layers of the layered structure. Each of the pairs ofoverlapping sub-structures that is in a different respective pair oflayers of the layered structure comprises first periodic components inwhich a difference in pitch is different, thereby providing a fringepattern having a different spatial frequency for each of the differentpairs of layers of the layered structure. In the example of FIG. 32, thetarget structures 91-94 respectively comprise first periodic componentshaving pitches P₁-P₄. Different pairs of the target structures havedifferent differences in pitch: for example, P₁−P₂≠P₁−P₃≠P₁−P₄ etc.Fringe patterns resulting from the different pairs thus have differentspatial frequencies and can therefore be resolved. The differentfrequencies allow the different fringe frequencies to encode differentinformation, for example a separate overlay value for each differentfringe frequency. Overlay errors with respect to different pairs oflayers in the target can therefore be obtained simultaneously via asingle illumination of one and the same region on the target. Detailedoverlay measurements are therefore possible without multiple differenttargets and/or multiple different measurement steps.

In the embodiment shown a plurality of reference structures R1-R3provide phase references for each of the pairs of target structuresbeing considered. Reference structure R1 acts as a phase reference withrespect to fringes formed by target structures 91 and 92 (e.g. byforming fringes having the same pitch as the fringes formed by thetarget structures 91 and 92). Reference structure R2 acts as a phasereference with respect to fringes formed by target structures 91 and 93(e.g. by forming fringes having the same pitch as the fringes formed bythe target structures 91 and 93). Reference structure R3 acts as phasereference with respect to fringes formed by target structures 91 and 93(e.g. by forming fringes having the same pitch as the fringes formed bythe target structures 91 and 93). Fewer or additional referencestructures may be provided as needed.

In any of the embodiments described above, each pair of targetsub-structures may comprise at least one target sub-structure with asecond periodic component orientated in a different direction to thefirst periodic component (i.e. either or both target sub-structures ineach pair each comprises such a second periodic component). For example,the second periodic component may be oriented perpendicularly to thesecond periodic component. The second periodic component acts toseparate in the pupil plane (Fourier space), independently from theFourier components that control the interference fringe phase andfrequency, the interfering predetermined diffraction orders from zerothorder scattered radiation, thereby improving the accuracy with which thescattered radiation formed by the interference between the predetermineddiffraction orders can be detected. Contamination from zeroth orderradiation is reduced. Additionally, the second periodic componentchanges the angle at which the scattered radiation formed by theinterference between the predetermined diffraction orders leaves thetarget 60 (see FIG. 9-11 for example). The second periodic component canthus be configured to ensure that the angle is appropriate for thedetection system of the metrology system (e.g. such that the scatteredradiation enters a pupil of the objective lens 16 of the detectionsystem and/or allowing the radiation to be directed to a particularlocation on a detector array of the detection system). The secondperiodic component therefore makes it possible simultaneously to have(a) symmetric doubly-diffracted orders within one plane and (b)separation of selected information-carrying orders from the zeroth orderin another, typically orthogonal, plane.

The target sub-structure with a second periodic component orientated ina different direction to the first periodic component may take variousforms, including one or more of the following: a checkerboard patternformed from square elements or rectangular elements, and a tiltedcheckerboard pattern formed from square elements or rectangular elementsrotated by a predetermined angle about an axis perpendicular to theplane of the checkerboard pattern.

Example checkerboard patterns formed from square elements are shown inFIGS. 14 and 20(b) for example. An example checkerboard pattern formedfrom rectangular (non-square) elements is shown in FIG. 34. An exampletilted checkerboard pattern is shown in FIG. 35. Checkerboard patternshave been found to work particularly effectively because of therelatively low level of unwanted harmonics in the diffraction pattern.The tilted checkerboard pattern may be favored over a regularcheckerboard pattern where it is desired to avoid corner-to-cornercontacts between elements of the checkerboard. However, other patternscould be used. A variant of the checkerboard pattern in which all of thesquare elements are aligned along both X and Y could be used forexample, as shown in FIG. 20(a). In an alternative embodiment, a pair oftarget sub-structures is provided in which a top grating has a pitch P₁and a bottom grating has a pitch P₂. Gratings lines on the bottomgrating are provided at an oblique angle theta relative to the topgrating. The bottom grating comprises multiple grating segments. A firstset of grating segments comprises grating lines rotated at +theta. Asecond set of grating segments comprises grating lines rotated at−theta. Alternatively or additionally, the segments from the differentsets may be interspersed with respect to each other to form a periodicpattern of equal and oppositely rotated grating segments. The rotatedsegments produce diffraction at the same angles as the checkerboard butwithout corner-to-corner contact of the individual structural elements.In a further alternative embodiment, a checkerboard pattern is formedwith rounded edges or unequal rectangle-space ratio.

An example of a typical two-dimensional grating is shown in FIG. 20(a)for example.

Further examples of particular target structures are shown in FIGS.36-39. In each of these figures, the pattern shown in (a) corresponds toa second target structure 92 (top target) and the pattern shown in (b)corresponds to a first target structure 91 (bottom target). The patternscould however be reversed.

FIG. 36(a) depicts a second target structure 92 having a footprintwithin 16 microns along Y (vertical in the figure) and 32 microns alongX (horizontal in the figure). These are purely exemplary dimensions.Target sub-structures 301 and 303 are line gratings with pitch (P₁)=450nm. Target sub-structures 302 and 304 are line gratings with pitch(P₂)=500 nm.

FIG. 36(b) depicts a first target structure 91 configured to form pairsof target sub-structures with the second target structure 92 of FIG.36(a). Target sub-structures 311 and 313 (which pair respectively withtarget sub-structures 301 and 303) comprise checkerboard patterns havinga pitch (P₂)=500 nm parallel to the pitches P₁ of target sub-structures301 and 303, and a pitch (P_(H))=500 nm in the perpendicular direction.Target substructures 312 and 314 (which pair respectively with targetsub-structures 302 and 304) comprise checkerboard patterns having apitch (P₁)=450 nm parallel to the pitches of target sub-structures 302and 304, and a pitch (P_(H))=500 nm in the perpendicular direction.Thus, P₁=450 nm and P₂=500 nm in this example. Target sub-structures301, 302, 311 and 312 provide sensitivity to overlay errors in thevertical direction in the figure. Target sub-structures 303, 304, 313and 314 provide sensitivity to overlay errors in the horizontaldirection in the figure.

FIG. 37 depicts a second target structure 92 and a first targetstructure 91 that are the same as in FIG. 36 except that the P₁=600 nm,P₂=700 nm and P_(H)=700 nm. The values of P₁, P₂ and P_(H) in FIGS. 36and 37 are purely exemplary.

FIG. 38(a) depicts a second target structure 92 having a footprint(purely exemplary) within 16 microns along Y (vertical in the figure)and 32 microns along X (horizontal in the figure). Target sub-structures301A and 301B comprise gratings having a parallel periodic componentwith a pitch (P₁)=450 nm. Target sub-structures 303A and 303B alsocomprise gratings having a parallel periodic component with a pitch(P₁)=450 nm (perpendicular to 301A and 301B). Outer gratings 301A and303A additionally comprise a periodicity of pitch (P_(H))=500 nm in aperpendicular direction.

Target sub-structures 302A and 302B comprise gratings having a parallelperiodic component with a pitch (P₂)=500 nm. Target sub-structures 304Aand 304B also comprise gratings having a parallel periodic componentwith a pitch (P₂)=500 nm (perpendicular to 302A and 302B). Outergratings 302A and 304A additionally comprise a periodicity of pitch(P_(H))=500 nm in a perpendicular direction.

The two-dimensional structure of the outer gratings 301A, 302A, 303A and304A produce twice the fringe period of the inner gratings 301B, 302B,303B and 304B. Doubling the fringe period can be desirable for reducingphase ambiguity, e.g. in the case that the overlay causes the inner setof fringes (produced by the inner gratings) to have a π phase shift, theouter set of fringes (produced by the outer gratings) would have a π/2phase shift. The range of unambiguous measurement is thereby increased.

FIG. 38(b) depicts a first target structure 91 configured to form pairsof target sub-structures with the second target structure 92 of FIG.38(a). Target substructures 311 and 313 comprise checkerboard patternshaving a pitch (P₂)=500 nm parallel to the pitches (P₁) of targetsub-structures 301A, 301B, 303A and 303B, and a pitch (P_(H))=500 nm inthe perpendicular direction. Target substructures 312 and 314 comprisecheckerboard patterns having a pitch (P₁)=450 nm parallel to the pitches(P₂) of target sub-structures 302A, 302B, 304A and 304B, and a pitch(P_(H))=500 nm in the perpendicular direction.

FIG. 39(a) depicts a second target structure 92 having a (purelyexemplary) footprint within 16 microns along Y (vertical in the figure)and 32 microns along X (horizontal in the figure). Target sub-structures321-324 comprise line gratings with the same pitch (P)=500 nm butdifferent overlay biases. The overlay biases are given by −P/8−d, P/8+d,−P/8+d and P/8−d, where d=20 nm, yielding overlay biases of −82.5 nm,−42.5 nm, 42.5 nm and 82.5 nm. Target sub-structures 321-324 providesensitivity to overlay error in the Y direction (in combination with thefirst target structure 91). Target sub-structures 325-328 are orientatedperpendicularly to the target sub-structures 321-324 but are otherwisethe same as the target sub-structures 321-324. Target sub-structures325-328 thus provide sensitivity to overlay error in the X direction (incombination with the first target structure 91).

FIG. 39(b) depicts a first target structure 91 configured to form pairsof target sub-structures with the second target structure 92 of FIG.39(a). Target substructure 330 comprises a checkerboard pattern having apitch (P)=500 nm parallel to the pitches (P) of target sub-structures321, 322, 323 and 324, and a pitch (P_(H))=500 nm in the perpendiculardirection (i.e. a checkerboard pattern comprising square elements). Thecheckerboard pattern also provides a pitch (P)=500 nm parallel to thepitches (P) of target sub-structures 325, 326, 327 and 328, and a pitch(P_(H))=500 nm in the perpendicular direction.

In an embodiment, a metrology apparatus is provided that is operable toperform any of the methods of measuring a target described herein. Themetrology apparatus may be configured as described above in FIG. 3(a)for example. An illumination system asymmetrically illuminates withmeasurement radiation a target 60 produced using a lithographic processon a substrate. A detection system detects scattered radiation arisingfrom illumination of the target 60. The detection system may comprise atleast the second measurement branch depicted in FIG. 3(a). The detectionsystem may comprise a sensor 23 that detects the scattered radiation,that may be formed by interference between the predetermined diffractionorders.

Conventional methods of extracting overlay are sensitive to path lengthdifferences between the diffracted waves (an effect known as the swingcurve), requiring extensive calibration and recipe optimization forevery new layer. The approach described herein offers an improvement topath length sensitivity. The waves observed in the dark field havediffracted twice, in such a way to minimize/eliminate the path lengthdifferences, and thus the swing curve. In other words, the aim is tohave the targets approximate a common-path interferometer. Measuringoverlay using the targets described herein, either from intensities ofpads or from Moire fringes (“phase-based”), is easier and more stablethan conventional dark-field diffraction-based overlay (μDBO)approaches. It is also robust against thickness variations over thewafer. It also allows for broadband illumination (essentially inherentwavelength multiplexing), bringing the advantages of multi-wavelengthacquirements in a single shot, and be useful at thicker stacks thanregular conventional dark-field diffraction-based overlay (μDBO)approaches.

However, the approach described herein may still suffer from apath-length sensitivity when used with an angular-resolved scatterometerwhich illuminates a target from a range of angles. This path-lengthsensitivity arises from there being too many double-diffracted waves,which have different path lengths. This problem is described below withreference to FIGS. 40 to 49. A solution is described with reference toFIGS. 50 to 55.

In FIGS. 40-49, the following elements are illustrated.

Top T and bottom B overlapping target sub-structures are shown inphysical cross sections of Figured 40 and 41. As illustrated for examplein FIGS. 43 and 44, the sub-structures are separated by an interlayer ofthickness D. The top sub-structure T (or T₁, or T₂) is a linear gratingwith pitch p₁ in the x-direction 2. The bottom sub-structure B is acheckerboard grating with pitch p_(h) in the y-direction 9 and p₂ in thex-direction x. In this example, p₂<p₁.

In FIGS. 42 to 44, 46, 47, 50 and 51, the interfaces at whichdiffraction happens are labelled as T₁, B and T₂ as rays are transmittedthrough the top sub-structure T at T₁, then reflected from the bottomsubstructure B and then transmitted through the top sub-structure Tagain at T₂.

Incoming and specularly reflected zeroth order rays are labelled as u,and in FIG. 40 as parallel rays u₁ and u₂.

Singly diffracted rays, which are incoming with respect to the bottomsub-structure B, having been diffracted in transmission at T₁ arelabelled v₁ and v₂. Singly diffracted rays, which are outgoing withrespect to the bottom sub-structure B, having been diffracted inreflection at B, are labelled v₃ and v₄. Non-propagating rays arelabelled v₁′ and v₃′.

Doubly diffracted rays, which are outgoing with respect to the bottomsub-structure B, having had their second diffraction in reflection at Bare labelled w₁ and w₂. Doubly diffracted rays, which are also outgoingwith respect to the bottom sub-structure B, having had their seconddiffraction in transmission at T₂, are labelled w₃ and w₄. In the 3Drepresentations, outgoing rays are shown being incident on a screen SCR.

In the target design disclosed above, in general a total of fourcoherent waves propagate to the sensor, as shown in FIGS. 40 to 42.Incoming light u diffracts at the top grating T₁, then at the bottomgrating B (which is a checkerboard in this example), and finally at thetop grating again T₂. Only the optical paths exiting the target at lowNA (numerical aperture) are drawn, since these are the ones the pass the0.4 NA pupil stop NA_(ps) of the example scatterometer. In this example,the pupil stop defines a detection aperture like that shown as 21 a inFIG. 3(a). This NA filtering is equivalently drawn in the pupil plane inFIGS. 45, 49, 52, 53(a), 54(a) and 55(a). In those figures, NA_(st)represents the propagation limit for waves inside the stack with indexof refraction higher than 1 (e.g. here 1.4). NA_(sn) represents the 0.95numerical aperture of the sensor.

The four outgoing waves are labeled w₁₋₄. In FIGS. 40-55, since p₁ andp₂ are not equal, there is a small residual angle between the rays,which causes fringe formation on the screen placed after the target (orequivalently, a camera).

FIGS. 40(a)-(c) depict trajectories in 2D of example rays through atarget with double diffraction (a) in reflection then transmission (2waves), (b) in transmission then reflection (2 waves), and (c) in bothreflection/transmission then in transmission/reflection (4 waves).

FIG. 40(a) has the same features as FIG. 21, but labelled differently toaid description of the rays.

FIGS. 41(a)-(c) depict trajectories 2D of coincident example raysthrough a target with double diffraction (a) in reflection thentransmission, (b) in transmission then reflection, and (c) in bothreflection/transmission then in transmission/reflection.

FIGS. 42(a)-(c) depict trajectories through an unfolded path in 2D ofcoincident example rays with double diffraction (a) in reflection thentransmission, (b) in transmission then reflection, and (c) in bothreflection/transmission then in transmission/reflection. FIG. 42 is thusillustrating the trajectory of the same rays in FIG. 41, but without thereversal of direction caused by reflection at the bottom sub-structure.The trajectory of the rays then continues down the page. This has beendone to more clearly illustrate the steps of double diffraction.

FIGS. 43(a)-(c) depict trajectories in 3D of coincident example raysthrough an unfolded path with double diffraction (a) in reflection thentransmission, (b) in transmission then reflection, and (c) in bothreflection/transmission then in transmission/reflection.

FIGS. 44(a)-(c) depict trajectories 2D of coincident example raysthrough a target with double diffraction (a) in reflection thentransmission, (b) in transmission then reflection, and (c) in bothreflection/transmission then in transmission/reflection.

FIGS. 45(a)-(c) depict Fourier space representations of doublediffraction (a) by a chequerboard then a grating, (b) by a grating thena chequerboard, and (c) by both a chequerboard/grating then by agrating/chequerboard. The Fourier space representations in FIGS. 45, 49,52, 53(a), 54(a), and 55(a) illustrate the pupil plane of thescatterometer.

Each small circle in the FIGS. 45(a) to (c) represents an angle of aray, shown in the corresponding FIGS. 43 and 44(a) to (c) respectively.For example, FIG. 45(a) shows the incident ray u, which is incidentalong the plane perpendicular to the direction of periodicity{circumflex over (x)} of the top grating sub-structure T. Thus, the rayu lies on the imaginary line IL in the illumination pupil passingthrough the optical axis (the center of the concentric NA circles).

The checkerboard of the bottom grating sub-structure B has four diagonaldiffraction directions in Fourier space, as shown as 102 in FIG. 22.Thus, diffraction by the bottom grating sub-structure B is shown as adiagonal line. For example, in FIG. 45(a), the incoming ray u isdiffracted diagonally (in k-space) into the directions labelled v₃ andv₄. Diagonal arrows (not shown) pointing away from the small circlelabelled u, in the opposite direction to those arrows shown, result indiffracted rays (not shown) that are outside the numerical aperturecircles, so rays are not propagated.

The linear grating of the top sub-structure T causes diffraction that ishorizontal in the k-space representation, as shown as 101 in FIG. 22.Thus, in FIG. 45(a), the rays v₃ and v₄ are diffracted at T₂horizontally (in k-space) to the rays w₃ and w₄ respectively.

In FIG. 45(a), the end result of the double diffraction at B then T₂ isthat the outgoing rays w₃ and w₄ are at angles captured within the 0.4NA pupil stop NA_(ps).

FIGS. 43-45(b) show the corresponding views for double diffraction by agrating then a chequerboard. In FIG. 45(b), the end result of the doublediffraction at T₂ then B is that the outgoing rays w₂ and w₁ are atangles captured within the 0.4 NA pupil stop NA_(ps).

FIGS. 43-45(c) show the rays and diffraction illustrated in 43-45(a) and43-45(b), which happen simultaneously, combined into one set of Figures.In FIG. 45(c), the end result of the double diffraction at T₂ then B aswell as B then T₂ is that the outgoing rays W_(2,3) and w_(1,4) are atangles captured within the 0.4 NA pupil stop NA_(ps).

The checkerboard grating is useful because it filters out somediffraction orders while also providing diffraction in a directiondifferent from a linear grating. A linear grating with period p₁ andsegmentation p₂ (an ortho pitch grating) would diffract in all eightdirections in k-space with horizontal, vertical and diagonal arrows. Buta checkerboard only retains four of these directions due to additionalinterference. If an ortho pitch grating were to be used as the bottomgrating sub-structure, the incident ray u would then also diffractdirectly down in the FIG. 45(a) (as well as diagonally) which would giverise to a ray within the 0.4 NA pupil stop NA_(ps). This would greatlyreduce the contrast of the measurement. The checkerboard, with only thediagonal directions, enables a design where only waves that diffractedtwice w₃ and w₄ make it into the detection aperture of the pupil stopNA_(ps) in the center.

A 45-degree rotated grating diffracts like a normal grating, so with twodiffraction orders, along only one diagonal. If a 0-degree rotatedgrating were used as the top sub-structure, and a 45 degree-rotatedgrating as the bottom sub-structure, two optical paths could be formedthat propagate though the detection pupil-stop aperture, but theresulting interference pattern would not carry overlay information. Theadvantage of a checkerboard is that the collected rays have diffractedfrom the checkerboard along different diagonal axes, in order to putoverlay information in the image.

If the index of refraction is larger than 1 the waves should berefracted when entering and exiting the stack, so both at T₁ and T₂.Since in this example the effective NA of the stack NA_(st) is largerthan one (1.4), there should be refraction (shown as further bending ofthe rays at T₁ and T₂) to make the Figures consistent in this regard.However, refraction has not been drawn to avoid confusion. Drawn withoutrefraction like this, any change of direction indicates a diffraction,and keeping track of diffractions is important in explaining theembodiments. Including refraction would make the 2D figures overlycomplicated, and the 3D figures hard to follow. Also, physically,refraction does not change the method, it is basically a radialrescaling of all angles, so the illumination asymmetry has the sameeffects.

FIGS. 40 to 45 illustrate diffraction of incoming rays u that are in animaginary plane perpendicular to the direction of periodicity of the topsub-structure. The observed dark-field intensity comes from theinterference of these four waves, which thus consists of 4×4=16 terms.If the four path lengths are perfectly equal, for the used wavelengthsand incoming angles, we would find no swing curve, in all sixteenintensity terms.

However, it is important to consider a range of incoming angles since anangular resolved scatterometer illumination source can illuminate with acontinuum of waves at different angles. Consequently, the paths of thefour waves are not equal, resulting in residual swing-curve behavior forsome terms. This is illustrated in FIGS. 46 to 49.

FIG. 46 depicts trajectories through an unfolded path in 3D of examplerays coincident at an arbitrary angle θ_(xz1) in the xz plane withrespect to the imaginary plane (here the zy plane) perpendicular to thedirection of periodicity of the top grating (here x). The imaginaryplane is depicted by line IP which lies on the plane. The trajectorieshave double diffraction in both reflection/transmission then intransmission/reflection.

FIG. 47 depicts trajectories through an unfolded path in 2D of examplerays, coincident at an arbitrary angle θ_(xz1) in the xz plane, withdouble diffraction in both reflection/transmission then intransmission/reflection. FIG. 47 is a side view of the optical pathsfrom FIG. 46, in the plane of double diffraction. In the plane of singlediffraction, orthogonal to the one shown in FIG. 47, all four paths areequal since they all diffract only once, at the checkerboard bottomgrating B.

In FIG. 47, the four paths are shown inside the stack, for illuminationat an arbitrary angle θ_(xz1) in this plane (the general case forangularly resolved scatterometer illumination). Clearly, paths endingwith w₄ and w₂ are nearly equal, nearly forming a parallelogram insidethe stack, and the same holds for paths ending with w₁ and w₃. However,the combination of longer and shorter path length rays leads to asensitivity to stack thickness.

FIG. 49 depicts a Fourier space representation of double diffraction,with illumination at an arbitrary angle θ_(xz1) in the xz plane, by botha chequerboard/grating then by a grating/chequerboard.

FIG. 48 is a graph of the path lengths of different rays as a functionof incidence angle θ (in degrees). Path-length differences □_(nm)between rays labelled w_(n) and w_(m) may be defined (where n,m=1 . . .4). The path length differences □₁₃, □₂₁ and □₂₄, are illustrated inFIG. 48, over a range of incoming angles, expressed in units of stackthickness, for wavelength shorter than the used pitches (□<p) and longerthan the used pitches (□>p). Clearly, for many angles and wavelengths,paths lengths are not equal. If the path length difference varies bymore than the wavelength, this will cause loss of contrast and artefactsin the dark-field image. Depending on which wavelengths and angles areused, as well as the diffraction properties of the gratings and stackthickness, this can mean that swing curve terms dominate intensity,reducing the benefits afforded by double diffraction described herein.It is apparent from FIG. 48 that □₂₄ is close to zero across the wholerange of incidence angle θ. Therefore, if just the rays w₂ and w₄ wereallowed to propagate then the stack thickness and swing curve effectscould be greatly reduced.

The path length problem can be mitigated by illuminating the targetasymmetrically (i.e. more from one side than the other) in thedouble-diffraction plane, so that only two of the four waves (e.g. onlyw₂ and w₄ in FIG. 47). In effect this frustrates propagation of rays w₁and w₃, which are sources of unwanted path length variation as afunction of incidence angle, to leave w₂ and w₄, which have more similarpath lengths. By doing this, only the □₂₄ curves in FIG. 48 are left,with by far the lowest path length difference. If the pitch in top andbottom grating is equal (which means there are no fringes forphase-based diffraction-based overlay measurement, but overlay isextracted from mean biased-pad intensities like in conventionalintensity-based diffraction-based overlay), the two optical paths areequal, and there is no swing curve due to path length remaining (i.e. itis a true common-path interferometer).

An example of the design principle (for phase-based diffraction-basedoverlay measurement), in a metrology apparatus with illumination from asingle quadrant, is shown in FIGS. 50 to 55.

FIG. 50 depicts trajectories through an unfolded path in 3D of examplerays coincident at an illumination angle θ_(xz1) in the xz planeconfigured to limit propagation of double diffraction in bothreflection/transmission then in transmission/reflection.

FIG. 51 depicts trajectories through an unfolded path in 2D of examplerays, coincident at an illumination angle θ_(xz1) in the xz planeconfigured to limit propagation of double diffraction in bothreflection/transmission then in transmission/reflection.

FIG. 52 depicts a Fourier space representation of double diffraction,with an illumination angle in the xz plane configured to limitpropagation by both a chequerboard/grating then by agrating/chequerboard. Although there is sufficient tilt of the incidenceangle θ_(xz1) to limit propagation of the long-path rays (byfrustrating—propagation of v₁′ and v₃′), it can be seen from FIG. 52that there is still a problem with detection of the double-diffractedrays. The ray labelled w₂ is outside of the pupil stop numericalaperture and detection aperture NA_(ps). This means ray w₂ will notreach the sensor. This problem may be overcome as illustrated by FIGS.53 to 55. FIG. 53 shows asymmetric illumination from a point andcorrection for the tilt by shifting the detection region by a distance Hin the pupil plane. In FIG. 54 another way of shifting the detectionregion is shown using a quadrant detection aperture. In FIG. 55 thecorrection for tilt is made by deflecting the detection region using a“quad wedge” in the detection branch.

FIG. 53(a) depicts a Fourier space representation of double diffraction,with an illumination angle in the xz plane configured to limitpropagation by both a chequerboard/grating then by agrating/chequerboard to an offset detection aperture NA_(ps). FIG. 53(b)depicts an illumination profile 532 and offset detection aperture 534corresponding to the Fourier space representation of FIG. 53(a).

FIG. 54(a) depicts a Fourier space representation of double diffraction,with an illumination angle in the xz plane configured to limitpropagation by both a chequerboard/grating then by agrating/chequerboard to a quadrant detection aperture. FIG. 54(a)corresponds to FIG. 52, but instead of rays, u, v and w representregions of the illumination profile and diffracted regions. FIG. 54(b)depicts an illumination profile 542 and quadrant detection aperture 544corresponding to the Fourier space representation of FIG. 54(a).Similarly, square regions are shown in FIG. 55(a).

FIG. 55(a) depicts a Fourier space representation of double diffraction,with an illumination angle in the xz plane configured to limitpropagation by both a chequerboard/grating then by agrating/chequerboard to a quad wedge. FIG. 55(b) depicts a square,asymmetric illumination profile 552 and quad wedge 554 (and 556 in sideelevation) corresponding to the Fourier space representation of FIG.55(a).

In FIG. 55(a), the sensor pupil is shown, with the four quadrants due toa “quad wedge” (such as 21 b in FIG. 3) labelled Q1-Q4. Like in FIGS.45, 49, 52, 53(a) and 54(a), the dash-dotted line indicates theeffective propagation limit NA_(st) for the waves propagating inside thestack (which are shown in FIG. 51 between the T₁ and T₂ interfaces).Since the NAs of the incoming waves are shifted by H with respect to thepupil center, waves v₄ and v₂ in the stack diffracting via Q3 and Q4respectively can propagate, but waves v₁ and v₃ diffracting in theopposite direction do not, since they are outside the effective NA.

An embodiment comprises a method of measuring a target formed by alithographic process, the target comprising a layered structure having afirst target structure (B) in a first layer and a second targetstructure (T) in a second layer. The method begins with positioning thetarget in an optical axis of an optical system having an illuminationpupil and a detection pupil in corresponding pupil planes of the opticalsystem.

FIGS. 50 to 55 illustrate illuminating the target with measurementradiation u at an illumination angle θ_(xz1) in the xz plane using anillumination profile in the illumination pupil that is offset from, andin this example asymmetric about, an imaginary line IL ({circumflex over(k)}_(x)=0 in FIG. 52) in the illumination pupil passing through theoptical axis (the center of the circles in FIGS. 52 to 55). Theimaginary line IL corresponds to a direction of periodicity ({circumflexover (x)} in FIG. 50) of a target structure. The illumination profile isconfigured to allow propagation to a detection region of the detectionpupil of an allowed order of a predetermined diffraction order v₂ (orv₄) while limiting propagation to the detection region of an equal andopposite order v₁′ (or v₃′) of that predetermined diffraction order.Scattered radiation of plural allowed diffraction orders w₄ and w₂ aredetected. The allowed diffraction orders are generated by diffraction ofthe measurement radiation from the first target sub-structure T and aresubsequently diffracted from the second target sub-structure B. Thescattered radiation of plural allowed diffraction orders may be formedby interference between the plural allowed diffraction orders, in theway described with reference to FIGS. 12 to 18.

The illumination profile is configured such that a first of the alloweddiffraction orders w₄ is generated by diffraction of a reflected orderv₄ in reflection from the first target structure B while propagation tothe detection region of the reflected order's equal and opposite orderv₃′ is limited. The subsequent diffraction w₄ of the first of theallowed diffraction orders from the second target structure comprisesdiffraction in transmission (at T₂) through the second target structureT. A second of the allowed diffraction orders w₂ is generated bydiffraction of a transmitted order v₂ in transmission (at T₁) throughthe second target structure T while propagation to the detection regionof the transmitted order's equal and opposite order v₁′ is limited. Thesubsequent diffraction w₂ of the second of the allowed diffractionorders from the second target structure comprises diffraction inreflection from the first target structure B. The limited propagation inthis example is the frustration of propagation. This is shown as thedashed lines v₁′ and v₃′ in FIGS. 50 and 51. In FIG. 52, the limitationof propagation is shown by the small circles (representing rays) v₁′ andv₃′ being outside the effective stack NA, NA_(st).

As described above, a characteristic of the lithographic process iscalculated using the detected scattered radiation of the alloweddiffraction orders. The characteristic of the lithographic process inthis example comprises an overlay error between the first targetsub-structure and the second target sub-structure.

The target may comprise three or more pairs (e.g. 61-64 as describedwith reference to FIGS. 24 to 26) of overlapping target sub-structures(e.g. 151-158 as described with reference to FIGS. 24 to 26). Each pairof overlapping target sub-structures comprising a first targetsub-structure in the first target structure and a second targetsub-structure in the second target structure. Each of the first targetsub-structure and the second target sub-structure in each pair ofoverlapping target sub-structures comprises a first periodic componenthaving the same pitch and orientation. Each pair of overlapping targetsub-structures is provided with a different overlay bias.

The detected scattered radiation of the allowed diffraction orderscomprises a plurality of intensity sub-regions (as shown in FIGS.54-55), each intensity sub-region having spatially uniform intensity andbeing formed by measurement radiation diffracted from a differentrespective pair of the three or more pairs of target sub-structures. Thecalculating of the characteristic of the lithographic process may use alevel of intensity in each intensity sub-region to determine thecharacteristic of the lithographic process.

The predetermined diffraction orders are defined with respect todiffraction from the first periodic component in each pair of targetsub-structures.

The overlay biases may comprise one or more pairs of equal and oppositeoverlay biases. The three or more pairs of target sub-structures maycomprise four pairs of target sub-structures.

As described with reference to FIG. 27, the overlay biases may comprisethe following: −P/8−d, P/8+d, −P/8+d and P/8−d, where P is the pitch ofthe first periodic component and d is a predetermined constant.

As described above, the detected scattered radiation of plural alloweddiffraction orders may be formed by interference between the alloweddiffraction orders and may comprise a fringe pattern. In this case, thecalculation of the characteristic of the lithographic process maycomprise calculating an overlay error between the first target structureand the second target structure by extracting a phase of the fringepattern. The target comprises at least one pair of overlapping targetsub-structures, each pair of overlapping target sub-structurescomprising a first target sub-structure in the first target structureand a second target sub-structure in the second target structure. Eachof the first target sub-structure and the second target sub-structure ineach pair of overlapping target sub-structures may comprise a firstperiodic component having the same orientation and a different pitch.The detected scattered radiation formed by interference between theallowed diffraction orders may comprise a fringe pattern formed by eachpair of target sub-structures. The target may further comprise areference structure configured to provide a radiation pattern having thesame periodicity as the fringe pattern, wherein the reference structureis provided in such a way that there is substantially no positionalshift of fringes in the radiation pattern as a function of overlay errorbetween the first target structure and the second target structure.

As described above, the target may comprise at least a first pair ofoverlapping target sub-structures and a second pair of overlappingtarget sub-structures. In the first pair of overlapping targetsub-structures, the first periodic component of the first targetsub-structure has a first pitch and the first periodic component of thesecond target substructure has a second pitch. In the second pair ofoverlapping target sub-structures, the first periodic component of thefirst target sub-structure has the second pitch and the first periodiccomponent of the second target sub-structure has the first pitch.

As described above, the target may comprise one or more further targetstructures respectively in one or more further layers of the layeredstructure The target may comprise at least one pair of overlappingtarget sub-structures in each of a plurality of different respectivepairs of layers of the layered structure, wherein each of the pairs ofoverlapping sub-structures that is in a different respective pair oflayers of the layered structure comprises first periodic components inwhich a difference in pitch is different, thereby providing a fringepattern having a different spatial frequency for each of the differentpairs of layers of the layered structure.

As described above, either or both target sub-structures in each of thepairs of target sub-structures may each comprise a second periodiccomponent orientated in a different direction to the first periodiccomponent. The first periodic component may be orientatedperpendicularly to the second periodic component. The targetsub-structure with a second periodic component that is orientated in adifferent direction to the first periodic component may comprises one ormore of the following: a checkerboard pattern formed from squareelements (such as described with reference to FIG. 20(b)) or rectangularelements (such as described with reference to FIG. 34), and a tiltedcheckerboard pattern formed from square elements or rectangular elementsrotated by a predetermined angle about an axis perpendicular to theplane of the checkerboard pattern (such as described with reference toFIG. 35).

As described above, the target structure may comprise a first periodiccomponent and a second periodic component orientated in a differentdirection to the first periodic component. The first periodic componentmay be orientated perpendicularly to the second periodic component. Thetarget structure with a second periodic component that is orientated ina different direction to the first periodic component may comprise oneor more of the following: a checkerboard pattern formed from squareelements (such as described with reference to FIG. 20(b)) or rectangularelements (such as described with reference to FIG. 34), and a tiltedcheckerboard pattern formed from square elements or rectangular elementsrotated by a predetermined angle about an axis perpendicular to theplane of the checkerboard pattern (such as described with reference toFIG. 35).

An embodiment may comprise a metrology apparatus, such as described withreference to FIG. 3. The metrology apparatus comprises an illuminationsystem configured to illuminate with measurement radiation a targetproduced using a lithographic process on a substrate. The metrologyapparatus also comprises a detection system configured to detectscattered radiation arising from illumination of the target. Themetrology apparatus is operable to perform the method described withreference to FIGS. 50 to 52 above.

By asymmetrically illuminating in this double-diffraction target design,embodiments of the present invention add an improved level of pupilfiltering, creating a common-path interferometer-target. Embodimentsgreatly improve the potential for double-diffraction target designs,adding a layer of filtering that can mean the difference between successand failure of measurement, and opens the door for fundamentally bettertarget designs.

The category of double-diffraction targets that this invention provideslargely removes the need for recipe optimization (to mitigate stackthickness effects), brings the advantages of multi-wavelength insingle-shot measurements, and extends diffraction-based overlaymeasurements to thicker stacks.

The target design described with reference to FIGS. 1 to 39 isapplicable to an angular resolved scatterometer having a centrosymmetricpupil filter (a 0.4 NA low-pass filter), such as NA_(ps) shown in FIG.45. Diffraction orders which had only been diffracted into another orderzero or one times are at high NA, and do not pass to the dark-fieldcamera, allowing for the dedicated detection of doubly-diffracted waves.Other scatterometers, such as one described in U.S. Pat. No. 9,223,227B2mentioned above that has a “quad wedge”, however, do not share thissymmetry and may suffer from problems described below with reference toFIGS. 56 and 57.

FIG. 56(a) depicts a Fourier space representation of double diffraction,with a quadrant illumination profile configured to limit propagation byboth a chequerboard/grating then by a grating/chequerboard to a quadwedge. FIG. 56(b) depicts a quadrant illumination profile 562 and quadwedge 564 (and 566 in cross-section) corresponding to the Fourier spacerepresentation of FIG. 56(a).

In FIG. 56(a) the pupil is divided into four quadrants Q₁ to Q₄, whichare the quadrants of the wedge before the dark-field camera. The wavesare labelled in the same way as for FIG. 55(a). In Q₁, non-diffractedoutgoing waves u are shown. Intermediate waves v₄ and v₂ diffract to Q₃and Q₄ respectively inside the stack. The waves then diffract a secondtime, ending up at w₄ and w₂ in the detection quadrant Q₂. This is acore mechanism of double-diffraction targets. The orders are truncatedat the pupil edge at NA=1, shown by the large circle, for eachdiffraction. In this example, the stack index of refraction is also 1.The solid arrows indicate diffraction directions, and dashed arrows showthe two “effective” directions of diffraction, from starting wave u tofinal waves w₄ and w₂. They are the directions of the vector-sums of thetwo subsequent diffractions. The angle θ_(M) between these effectivedirections finally governs the Moiré fringes used for overlay detectionin phase-based μDBO, as described herein.

FIG. 56(a) illustrates a problem of phase-based detection using a quadwedge sensor; one of the two effective diffraction directions (righthand dashed arrow) puts a final diffraction order w₄ on top of thevertical dividing line between left and right quadrants, which is wherethis example optical system has an 0.1 NA safe zone to prevent lightfrom scattering uncontrollably from the wedge prism (indicated by thestar). If we were to make the pitch in the top and bottom structuresequal, θ_(M) becomes zero, and the final orders move horizontallytowards each other, avoiding the vertical safe zone. However, this meanswe can no longer detect overlay from the phase of fringes, but insteadneed to revert to mean pad intensity measurements, like for conventionalμDBO.

In addition to the problem described with relation to FIG. 56, anotherproblem can arise for scatterometers such as ones described in U.S. Pat.No. 9,223,227B2 mentioned above that have image copy-and-rotate devicesin the illumination system. As mentioned above, these devices provide a180-degree point symmetry function that symmetrizes the illuminationprofile around the optical axis. Therefore, in the example scatterometerdescribed here, with an image copy-and-rotate device any illumination inQ₁ is also present in Q₃ with 180-degree point symmetry. This isillustrated in FIG. 57(a), which depicts a Fourier space representationof just the first diffraction of the incident wave by the top grating,with a point symmetrized quadrant illumination profile. FIG. 57(b)depicts a point symmetrized quadrant illumination profile 572 and quadwedge 574, 576 corresponding to the Fourier space representation of FIG.57(a).

With reference to FIG. 57(a), the non-diffracted light u_(L) and u_(R)are respectively positioned in quadrants Q₁ and Q₃. The illuminationregion u_(R) will diffract into the 1^(st) order v₁ in quadrant Q₂,which is where the overlay-carrying light w₂ and w₄ shown in FIG. 56(a)also goes. This once-diffracted 1^(st) order wave v₁ is typically muchstronger in intensity than the twice-diffracted light that it isdesirable to detect. This creates a very strong background, ruining thecontrast of the measurement.

Thus, for double-diffraction (which includes phase-based) μDBO, thecombination of a safe zone and an image copy-and-rotate device makessome angular resolved scatterometers not suitable for the phase-basedtarget design described with reference to FIGS. 1 to 39 above. To solvethese problems, in an embodiment, the 1D grating-checkerboard ismodified by stretching it in one direction (x or y), and rotating itwith respect to the sensor by 45 degrees (either by printing a rotatedtarget, or by rotating the wafer by 45 degrees, or by rotatingcomponents of the illumination and detection systems). The reasoningbehind this is illustrated in FIGS. 58 to 62.

FIG. 58(a) depicts a Fourier space representation of the firstdiffraction of the incident wave by a stretched and 45-degree rotatedcheckerboard, with an offset illumination profile. FIG. 58(b) depicts astretched and 45-degree rotated checkerboard that causes diffractiondepicted in the Fourier space representation of FIG. 58(a). In FIG.58(a) the pupil is shown, using an 0.45 NA square aperture u. Twodiffractions into the first order v₄ and v₆ are shown, due to themodified checkerboard, which are maximally distinct (maximum angleθ_(max)), but do not hit a safe zone, and do not diffract into Q₂ or Q₄,which are detection quadrants. The two problems described above are usedto define design rules: “no diffraction overlaps with safe zones”, and“only diffractions that put non-diffracted light into another regionalready containing non-diffracted light”. These two rules togethernaturally lead to the diffraction orders shown in FIG. 58(a), which areat shallow angles with respect to the indicated x-axis, which runs at 45degrees from the pupil horizontal, requiring some minimum NA shift,labelled NA_(min). These diffraction orders are realized by thestretched checkerboard.

FIG. 59(a) depicts a Fourier space representation of double diffractionvia a stretched checkerboard, with an illumination profile configured tolimit propagation by both the chequerboard/grating then by thegrating/chequerboard to a 45-degree rotated quad wedge. FIG. 59(b)depicts a stretched checkerboard, rotated by 45 degrees with respect tothe quad wedge and thus a line of symmetry in the detection region, thatcauses diffraction depicted by diagonal arrows in the Fourier spacerepresentation of FIG. 59(a).

With reference to FIG. 59(a), diffraction orders for a complete targetare shown, rather than for just the checkerboard as shown in FIG. 58(a).The checkerboard shown in FIG. 59(b) is combined with a 1D grating inthe x-direction. The pupil has been rotated by 45 degrees with respectto FIG. 58(a) for clarity. The minimum NA shifts in x and y are shown asNA_(min,x)=1.41 and NA_(min,y)=0.78, derived from the dimensions of thescatterometer aperture, wedge, and pupil edge. The intermediate ordersv₂, v₄ and v₆, which propagate inside the stack before diffracting againinto the final orders are inside the “effective” numerical apertureNA_(st) which scales with the stack index of refraction (here 1.5), andthus they propagate to the detector. The final orders w₂, w₄, and w₆ areinside the 0.95 NA sensor pupil (solid circle), and thus they propagateto the sensor.

Like in FIG. 57(a), we can consider illumination from Q₃, due to animage copy-and-rotate device. FIGS. 60 to 62 illustrate the buildup ofthe total pupil illumination and diffraction and ray trajectories withan image copy-and-rotate device.

FIG. 60(a), reproduced from FIG. 59(a) but simplified for clarity andcomparison with FIGS. 61(a) and 62(a), depicts a Fourier spacerepresentation of double diffraction via a stretched checkerboard, withan illumination profile configured with a wave u_(L) to limitpropagation by both the chequerboard/grating then by thegrating/chequerboard to a 45-degree rotated quad wedge. FIG. 60(b)depicts trajectories through an unfolded path in 2D of the rays endingin quadrant Q₂ in the Fourier space representation of FIG. 60(a). FIG.60(b) is similar to FIG. 51, but the outgoing rays w₂ and w₄ areparallel with the incoming wave u_(L). Although FIG. 60(b) relates onlyto the waves arriving at quadrant Q₂, if v₃′ is replaced with v₅′ and v₄is replaced with v₆, it could relate to the waves arriving at Q₄. FIGS.61(b) and 62(b) could be similarly modified to relate to Q₄.

The image copy-and-rotate device creates a mirror image of FIG. 60(a)(flipped left-right), shown in FIG. 61(a). This illumination u_(R) addsin intensity to the dark-field image created by the diffraction ordersin FIG. 60(a), since the quadrants are made incoherent by the imagecopy-and-rotate device. FIG. 61(a), like FIG. 60(a), depicts a Fourierspace representation of double diffraction via a stretched checkerboard,but with a point symmetrized illumination profile compared to FIG.60(a). FIG. 61(b) depicts trajectories through an unfolded path in 2D ofthe rays ending in quadrant Q₂ in the Fourier space representation ofFIG. 61(a).

FIG. 62(a) depicts a Fourier space representation of double diffractionvia a stretched checkerboard, combining the illumination profiles ofboth FIGS. 60(a) and 61(a), with incoming waves u_(L) and u_(R). FIG.62(b) depicts the combined trajectories of FIGS. 60(b) and 61(b) throughan unfolded path in 2D of the rays ending in quadrant Q₂ in the Fourierspace representations of FIG. 62(a).

Thus, a second common-path interferometer is formed by the combination,and the signal from the two adds up instead of cancelling, halving anycamera integration time (so the image copy-and-rotate device provides anadvantage). In summary, due to a stretching and rotation operation ofthe target design, the required pupil parameters are provided for adouble-diffraction target which is suitable for scatterometers with quadwedges and image copy-and-rotate devices. However, there is no room forthe overlapping final orders w (for example w₂ and w₆) to move to thesides in the x-direction before they fall out of the 0.95 NA pupil(solid line circle) or cross a safe zone. For phase-based detection, wewould want to make the pitches in the top and bottom grating unequal,thereby moving the now perfectly-overlapping squares w₂ and w₄ (andsimilarly w₂ and w₆) away from each other. As a result, one square (e.g.w₂) would move left, and the other (e.g. w₄) would move right in FIG.60(a), like shown in FIG. 55(a). However, there is no room for thesquares w₂ to move to the left, since that is where the safe zone of thewedge lies (the black diagonal line where one does not want illuminationto fall since it may scatter unpredictably), and also where the edge ofthe pupil is (the limiting output NA). Thus, there is no room forphase-based detection, but only for intensity-based detection (i.e.θ_(M)=0, as opposed to FIG. 56(a) where θ_(M)>0).

In FIG. 62(a), there are thus eight squares labelled w; two pairs ofperfectly overlapping ones in Q₂, and two pairs of perfectly overlappingin Q₄. The wedge separates the light from Q₂ and Q₄, so that it isimaged onto a different camera CCD region, so the waves in Q₂ do notinterfere with those in Q₄. The interference between waves which givesintensity variations, only happens between the perfectly overlappingsquares (so one can roughly view what happens in Q₂ as totally separatefrom Q₄). This overlapping of waves is a general property of μDBO; whenthe 1^(st) order is detected on the pupil camera for a conventional μDBOmeasurement, that is actually the sum of four waves, which are perfectlyoverlapping in the pupil. The interference between those waves is whatgives us overlay information. The advantage of the arrangement shown inFIG. 62(a) is that the path length differences are eliminated, by thenon-propagation of the long path length rays, as shown by the dashedlines in FIG. 62(b).

To realize these pupil parameters in FIGS. 60 to 62, a checkerboardgrating which has been stretched in the y-direction is used. The ratioof y over x stretching is exactly the minimum NA shift in x, NA_(min,x),over the minimum NA shift in y, NA_(min,y), so the ratio of directionsis inversely proportional to the ratio of minimum NA shifts. Such acheckerboard is depicted in FIGS. 58(b) and 59(b), with the correctstretching in y of nearly a factor 2 (with 10 periods in both x and y).

This target has an important advantage, namely that there are only twowaves propagating to each detection quadrant instead of four, making ita fundamentally better common-path interferometer, compared to thearrangement described with reference to FIG. 49.

The embodiment described with reference to FIGS. 56 to 62 enables theuse of double-diffraction targets for scatterometers with imagecopy-and-rotate devices. No hardware change is required; simple45-degree rotations may be used, unless targets at 45-degree inclinationare printed. The embodiment is also applicable to scatterometers withoutimage copy-and-rotate devices and illumination from a single quadrant,to optimize bandwidth of the double-diffraction design.

The embodiment described with reference to FIGS. 56 to 62 can be used ina similar way to that described with reference to FIGS. 50 to 55. ThusFIGS. 56 to 62 illustrate illuminating the target with measurementradiation u at an illumination angle in the xz plane using anillumination profile in the illumination pupil that is offset from animaginary line IL ({circumflex over (k)}_(x)=0 in FIG. 59(a)) in theillumination pupil passing through the optical axis (the center of thecircles in FIG. 59(a)). The imaginary line IL corresponds to a directionof periodicity (the x-axis in FIG. 59(b)) of a target structure. Theillumination profile u is configured to allow propagation to a detectionregion of the detection pupil of an allowed order of a predetermineddiffraction order v₂ (or v₄ or v₆) while limiting propagation to thedetection region of an equal and opposite order v₁′ (or v₃′ or v₅′) ofthat predetermined diffraction order. The illumination profile us isalso configured to position the allowed diffraction orders in thedetection pupil in Q₂ and Q₄ separately from zero and once diffractedorders in Q₁ and Q₃. Scattered radiation of plural allowed diffractionorders w₄, w₆ and w₂ are detected. The allowed diffraction orders aregenerated by diffraction of the measurement radiation from the firsttarget sub-structure T and are subsequently diffracted from the secondtarget sub-structure B.

The target structure comprises a stretched checkerboard with firstperiodic component and a second periodic component orientated in adifferent direction (here perpendicular) to the first periodiccomponent. In this embodiment, the first periodic component isorientated at 45 degrees with respect to a line of symmetry in thedetection region (a diagonal line bisecting the pupil in FIG. 59(a)).

While the targets described above are metrology targets specificallydesigned and formed for the purposes of measurement, in otherembodiments, properties may be measured on targets which are functionalparts of devices formed on the substrate. Many devices have regular,grating-like structures. The terms ‘target grating’ and ‘target’ as usedherein do not require that the structure has been provided specificallyfor the measurement being performed. Further, pitch of the metrologytargets is close to the resolution limit of the optical system of thescatterometer, but may be much larger than the dimension of typicalproduct features made by lithographic process in the target portions C.In practice the lines and/or spaces of the overlay gratings within thetargets may be made to include smaller structures similar in dimensionto the product features.

In association with the physical grating structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing methods of measuring targets on a substrateand/or analyzing measurements to obtain information about a lithographicprocess. This computer program may be executed for example within unitPU in the apparatus of FIG. 3 and/or the control unit LACU of FIG. 2.There may also be provided a data storage medium (e.g., semiconductormemory, magnetic or optical disk) having such a computer program storedtherein. Where an existing metrology apparatus, for example of the typeshown in FIG. 3, is already in production and/or in use, the inventioncan be implemented by the provision of updated computer program productsfor causing a processor to perform the modified step S6 and so calculateoverlay error or other parameters with reduced sensitivity to structuralasymmetry.

The program may optionally be arranged to control the optical system,substrate support and the like to perform the steps S2-S5 formeasurement of asymmetry on a suitable plurality of targets.

While the embodiments disclosed above are described in terms ofdiffraction based overlay measurements (e.g., measurements made usingthe second measurement branch of the apparatus shown in FIG. 3(a)), inprinciple the same models can be used for pupil based overlaymeasurements (e.g., measurements made using the first measurement branchof the apparatus shown in FIG. 3(a)). Consequently, it should beappreciated that the concepts described herein are equally applicable todiffraction based overlay measurements and pupil based overlaymeasurements.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

Further embodiments according to the present invention are furtherdescribed in below numbered clauses:

1. A method of measuring a target formed by a lithographic process, thetarget comprising a layered structure having a first target structure ina first layer and a second target structure in a second layer, themethod comprising:

positioning the target in an optical axis of an optical system having anillumination pupil and a detection pupil in corresponding pupil planesof the optical system,

illuminating the target with measurement radiation using an illuminationprofile in the illumination pupil that is offset from an imaginary linein the illumination pupil passing through the optical axis, theimaginary line corresponding to a direction of periodicity of a targetstructure, wherein the illumination profile is configured to allowpropagation to a detection region of the detection pupil of an allowedorder of a predetermined diffraction order while limiting propagation tothe detection region of an equal and opposite order of thatpredetermined diffraction order;

detecting scattered radiation of plural allowed diffraction orders,wherein the allowed diffraction orders are generated by diffraction ofthe measurement radiation from the first target structure and aresubsequently diffracted from the second target structure; and

calculating a characteristic of the lithographic process using thedetected scattered radiation of the allowed diffraction orders.

2. The method of clause 1, wherein the illumination profile isconfigured to position the allowed diffraction orders in the detectionpupil separately from zero and once diffracted orders.

3. The method of clause 1 or clause 2, wherein the scattered radiationof plural allowed diffraction orders is formed by interference betweenthe plural allowed diffraction orders.

4. The method of any of clauses 1-3, wherein the illumination profile isconfigured such that:

a first of the allowed diffraction orders is generated by diffraction ofa reflected order in reflection from the first target structure whilepropagation to the detection region of the reflected order's equal andopposite order is limited and the subsequent diffraction of the first ofthe allowed diffraction orders from the second target structurecomprises diffraction in transmission through the second targetstructure; anda second of the allowed diffraction orders is generated by diffractionof a transmitted order in transmission through the second targetstructure while propagation to the detection region of the transmittedorder's equal and opposite order is limited and the subsequentdiffraction of the second of the allowed diffraction orders from thesecond target structure comprises diffraction in reflection from thefirst target structure.5. The method of any preceding clause, wherein:

the target comprises three or more pairs of overlapping targetsub-structures, each pair of overlapping target sub-structurescomprising a first target sub-structure in the first target structureand a second target sub-structure in the second target structure;

each of the first target sub-structure and the second targetsub-structure in each pair of overlapping target sub-structurescomprises a first periodic component having the same pitch andorientation; and each pair of overlapping target sub-structures isprovided with a different overlay bias.

6. The method of clause 5, wherein the detected scattered radiation ofthe allowed diffraction orders comprises a plurality of intensitysub-regions, each intensity sub-region having spatially uniformintensity and being formed by measurement radiation diffracted from adifferent respective pair of the three or more pairs of targetsub-structures, and wherein the calculating of the characteristic of thelithographic process uses a level of intensity in each intensitysub-region to determine the characteristic of the lithographic process.7. The method of clause 5 or 6, wherein the predetermined diffractionorders are defined with respect to diffraction from the first periodiccomponent in each pair of target sub-structures.8. The method of any of clauses 5-7, wherein the overlay biases compriseone or more pairs of equal and opposite overlay biases.9. The method of any of clauses 5-8, wherein the three or more pairs oftarget sub-structures comprises four pairs of target sub-structures.10. The method of clause 9, wherein the overlay biases comprise thefollowing: −P/8−d, P/8+d, −P/8+d and P/8−d, where P is the pitch of thefirst periodic component and d is a predetermined constant.11. The method of any of clauses 1-4, wherein the detected scatteredradiation of plural allowed diffraction orders is formed by interferencebetween the allowed diffraction orders and comprises a fringe pattern.12. The method of clause 11, wherein the calculation of thecharacteristic of the lithographic process comprises calculating anoverlay error between the first target structure and the second targetstructure by extracting a phase of the fringe pattern.13. The method of clause 11 or 12, wherein:

the target comprises at least one pair of overlapping targetsub-structures, each pair of overlapping target sub-structurescomprising a first target sub-structure in the first target structureand a second target sub-structure in the second target structure; and

each of the first target sub-structure and the second targetsub-structure in each pair of overlapping target sub-structurescomprises a first periodic component having the same orientation and adifferent pitch.

14. The method of clause 13, wherein the detected scattered radiationformed by interference between the allowed diffraction orders comprisesa fringe pattern formed by each pair of target sub-structures.

15. The method of clause 14, wherein the target further comprises areference structure configured to provide a radiation pattern having thesame periodicity as the fringe pattern, wherein the reference structureis provided in such a way that there is substantially no positionalshift of fringes in the radiation pattern as a function of overlay errorbetween the first target structure and the second target structure.16. The method of any of clauses 13-15, wherein:the target comprises at least a first pair of overlapping targetsub-structures and a second pair of overlapping target sub-structures;in the first pair of overlapping target sub-structures, the firstperiodic component of the first target sub-structure has a first pitchand the first periodic component of the second target substructure has asecond pitch; andin the second pair of overlapping target sub-structures, the firstperiodic component of the first target sub-structure has the secondpitch and the first periodic component of the second targetsub-structure has the first pitch.17. The method of any of clauses 11-16, wherein:the target comprises one or more further target structures respectivelyin one or more further layers of the layered structure;the target comprises at least one pair of overlapping targetsub-structures in each of a plurality of different respective pairs oflayers of the layered structure, wherein each of the pairs ofoverlapping sub-structures that is in a different respective pair oflayers of the layered structure comprises first periodic components inwhich a difference in pitch is different, thereby providing a fringepattern having a different spatial frequency for each of the differentpairs of layers of the layered structure.18. The method of any of clauses 5-17, wherein either or both targetsub-structures in each of the pairs of target sub-structures eachcomprises a second periodic component orientated in a differentdirection to the first periodic component.19. The method of clause 18, wherein the first periodic component isorientated perpendicularly to the second periodic component.20. The method of clause 18 or 19, wherein the first periodic componentis orientated at 45 degrees with respect to a line of symmetry in thedetection region.21. The method of any of clauses 18-20, wherein the target sub-structurewith a second periodic component orientated in a different direction tothe first periodic component comprises one or more of the following: acheckerboard pattern formed from square elements or rectangularelements, and a tilted checkerboard pattern formed from square elementsor rectangular elements rotated by a predetermined angle about an axisperpendicular to the plane of the checkerboard pattern.22. The method of any of clauses 1-4, wherein a target structurecomprises a first periodic component and a second periodic componentorientated in a different direction to the first periodic component.23. The method of clause 22, wherein the first periodic component isorientated perpendicularly to the second periodic component.24. The method of clause 22 or 23, wherein the first periodic componentis orientated at 45 degrees with respect to a line of symmetry in thedetection region.25. The method of any of clauses 22-24, wherein the target structurewith a second periodic component orientated in a different direction tothe first periodic component comprises one or more of the following: acheckerboard pattern formed from square elements or rectangularelements, and a tilted checkerboard pattern formed from square elementsor rectangular elements rotated by a predetermined angle about an axisperpendicular to the plane of the checkerboard pattern.26. The method of any preceding clause, wherein said characteristic ofthe lithographic process comprises an overlay error between the firsttarget structure and the second target structure.27. A metrology apparatus comprising:

an illumination system configured to illuminate with measurementradiation a target produced using a lithographic process on a substrate;and

a detection system configured to detect scattered radiation arising fromillumination of the target, wherein:

the metrology apparatus is operable to perform the method of any ofclauses 1-26.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. A method of measuring a target formed by alithographic process, the target comprising a layered structure having afirst target structure in a first layer and a second target structure ina second layer, the method comprising: positioning the target in anoptical axis of an optical system having an illumination pupil and adetection pupil in corresponding pupil planes of the optical system;illuminating the target with measurement radiation using an illuminationprofile in the illumination pupil that is offset from an imaginary linein the illumination pupil passing through the optical axis, theimaginary line corresponding to a direction of periodicity of a targetstructure, wherein the illumination profile allows propagation to adetection region of the detection pupil of an allowed order of apredetermined diffraction order while limiting propagation to thedetection region of an equal and opposite order of that predetermineddiffraction order; detecting scattered radiation of plural alloweddiffraction orders, wherein the allowed diffraction orders are generatedby diffraction of the measurement radiation from the first targetstructure and are subsequently diffracted from the second targetstructure; and calculating a characteristic of the lithographic processusing the detected scattered radiation of the allowed diffractionorders, wherein: a first of the allowed diffraction orders is generatedby diffraction of a reflected order in reflection from the first targetstructure while propagation to the detection region of the reflectedorder's equal and opposite order is limited and the subsequentdiffraction of the first of the allowed diffraction orders from thesecond target structure comprises diffraction in transmission throughthe second target structure; and a second of the allowed diffractionorders is generated by diffraction of a transmitted order intransmission through the second target structure while propagation tothe detection region of the transmitted order's equal and opposite orderis limited and the subsequent diffraction of the second of the alloweddiffraction orders from the second target structure comprisesdiffraction in reflection from the first target structure.
 2. The methodof claim 1, wherein the illumination profile positions the alloweddiffraction orders in the detection pupil separately from zero and oncediffracted orders.
 3. The method of claim 1, wherein the scatteredradiation of plural allowed diffraction orders is formed by interferencebetween the plural allowed diffraction orders.
 4. The method of claim 1,wherein: the target comprises three or more pairs of overlapping targetsub-structures, each pair of overlapping target sub-structurescomprising a first target sub-structure in the first target structureand a second target sub-structure in the second target structure; eachof the first target sub-structure and the second target sub-structure ineach pair of overlapping target sub-structures comprises a firstperiodic component having the same pitch and orientation; and each pairof overlapping target sub-structures is provided with a differentoverlay bias.
 5. The method of claim 4, wherein: the detected scatteredradiation of the allowed diffraction orders comprises a plurality ofintensity sub-regions, each intensity sub-region having spatiallyuniform intensity and being formed by measurement radiation diffractedfrom a different respective pair of the three or more pairs of targetsub-structures, and the calculating of the characteristic of thelithographic process uses a level of intensity in each intensitysub-region to determine the characteristic of the lithographic process.6. The method of claim 4, wherein the predetermined diffraction ordersare defined with respect to diffraction from the first periodiccomponent in each pair of target sub-structures.
 7. The method of claim4, wherein the overlay biases comprise one or more pairs of equal andopposite overlay biases.
 8. The method of claim 4, wherein the three ormore pairs of target sub-structures comprises four pairs of targetsub-structures.
 9. The method of claim 8, wherein the overlay biasescomprise the following: −P/8−d, P/8+d, −P/8+d and P/8−d, where P is thepitch of the first periodic component and d is a predetermined constant.10. The method of claim 1, wherein the detected scattered radiation ofplural allowed diffraction orders is formed by interference between theallowed diffraction orders and comprises a fringe pattern.
 11. Themethod of claim 10, wherein the calculation of the characteristic of thelithographic process comprises calculating an overlay error between thefirst target structure and the second target structure by extracting aphase of the fringe pattern.
 12. The method of claim 10, wherein: thetarget comprises at least one pair of overlapping target sub-structures,each pair of overlapping target sub-structures comprising a first targetsub-structure in the first target structure and a second targetsub-structure in the second target structure; and each of the firsttarget sub-structure and the second target sub-structure in each pair ofoverlapping target sub-structures comprises a first periodic componenthaving the same orientation and a different pitch.
 13. The method ofclaim 1, wherein a target structure comprises a first periodic componentand a second periodic component orientated in a different direction tothe first periodic component.
 14. A metrology apparatus comprising: anillumination system configured to illuminate with measurement radiationa target produced using a lithographic process on a substrate; and adetection system configured to detect scattered radiation arising fromillumination of the target, wherein: the metrology apparatus is operableto measure a target formed by a lithographic process, the targetcomprising a layered structure having a first target structure in afirst layer and a second target structure in a second layer by:positioning the target in an optical axis of an optical system having anillumination pupil and a detection pupil in corresponding pupil planesof the optical system; illuminating the target with measurementradiation using an illumination profile in the illumination pupil thatis offset from an imaginary line in the illumination pupil passingthrough the optical axis, the imaginary line corresponding to adirection of periodicity of a target structure, wherein the illuminationprofile is configured to allow propagation to a detection region of thedetection pupil of an allowed order of a predetermined diffraction orderwhile limiting propagation to the detection region of an equal andopposite order of that predetermined diffraction order; detectingscattered radiation of plural allowed diffraction orders, wherein theallowed diffraction orders are generated by diffraction of themeasurement radiation from the first target structure and aresubsequently diffracted from the second target structure; andcalculating a characteristic of the lithographic process using thedetected scattered radiation of the allowed diffraction orders; thetarget comprises three or more pairs of overlapping targetsub-structures, each pair of overlapping target sub-structurescomprising a first target sub-structure in the first target structureand a second target sub-structure in the second target structure; eachof the first target sub-structure and the second target sub-structure ineach pair of overlapping target sub-structures comprises a firstperiodic component having the same pitch and orientation; and each pairof overlapping target sub-structures is provided with a differentoverlay bias.
 15. The metrology apparatus of claim 14, wherein: thedetected scattered radiation of the allowed diffraction orders comprisesa plurality of intensity sub-regions, each intensity sub-region havingspatially uniform intensity and being formed by measurement radiationdiffracted from a different respective pair of the three or more pairsof target sub-structures; and the calculating of the characteristic ofthe lithographic process uses a level of intensity in each intensitysub-region to determine the characteristic of the lithographic process.16. The metrology apparatus of claim 14, wherein the predetermineddiffraction orders are defined with respect to diffraction from thefirst periodic component in each pair of target sub-structures.
 17. Themetrology apparatus of claim 14, wherein the overlay biases comprise oneor more pairs of equal and opposite overlay biases.
 18. The metrologyapparatus of claim 14, wherein the three or more pairs of targetsub-structures comprises four pairs of target sub-structures.
 19. Themetrology apparatus of claim 18, wherein the overlay biases comprise thefollowing: −P/8−d, P/8+d, −P/8+d and P/8−d, where P is the pitch of thefirst periodic component and d is a predetermined constant.
 20. A methodof measuring a target formed by a lithographic process, the targetcomprising a layered structure having a first target structure in afirst layer and a second target structure in a second layer, the methodcomprising: positioning the target in an optical axis of an opticalsystem having an illumination pupil and a detection pupil incorresponding pupil planes of the optical system; illuminating thetarget with measurement radiation using an illumination profile in theillumination pupil that is offset from an imaginary line in theillumination pupil passing through the optical axis, the imaginary linecorresponding to a direction of periodicity of a target structure,wherein the illumination profile allows propagation to a detectionregion of the detection pupil of an allowed order of a predetermineddiffraction order while limiting propagation to the detection region ofan equal and opposite order of that predetermined diffraction order;detecting scattered radiation of plural allowed diffraction orders,wherein the allowed diffraction orders are generated by diffraction ofthe measurement radiation from the first target structure and aresubsequently diffracted from the second target structure; andcalculating a characteristic of the lithographic process using thedetected scattered radiation of the allowed diffraction orders, whereina target structure comprises a first periodic component and a secondperiodic component orientated in a different direction to the firstperiodic component.